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GENETICS 

IN 

EELATION TO AGRICULTURE 



BY 
ERNEST BROWN BABCOCK 

PROFESSOR OF GENETICS, UNIVERSITY OF CALIFORNIA 

AND 

ROY EL WOOD CLAUSEN 

ASSISTANT PROFESSOR OF GENETICS, UNIVERSITY OF CALIFORNIA 



First Edition 



McGRAW-HILL BOOK COMPANY, Inc. 

239 WEST 39TH STREET. NEW YORK 



LONDON: HILL PUBLISHING CO., Ltd. 

6 & 8 BOUVERIE ST., E. C. 

1918 



o. 



-? 



Copyright, 1918, by the 
McGraw-Hill Book Company, Inc. 



APR 22 1918 



THIS MAPLE FKESS TOKK PA. 









THOMAS HUNT MORGAN 



PREFACE 

Of all the sciences that contribute to the great, tertiary' composite 
which is known as agriculture none is more important economically than 
genetics. One may not overlook the fundamental relation borne by the 
primary sciences, mathematics, physics and chemistry, and by the second- 
ary sciences, botany, zoology, geology, meteorology and economics, to 
the production and distribution of raw materials. But we confidently 
assert that the science which underlies the improvement of plants and 
animals for agricultural purposes is destined to receive increasing atten- 
tion is agricultural education and in agricultural practice. Without 
doubt vast possibilities await realization through the more thorough and 
systematic development of our living economic resources. Such de- 
velopment is directly dependent upon the successful utilization of genetic 
principles in plant and animal breeding. The science of genetics is still 
very young, but it is firmly established and is developing rapidly. It 
claims the attention of the producer of today and invites the most serious 
study of the agriculturist of tomorrow. It lays claim also to the interest 
of the eugenist, the sociologist and the philanthropist and all students 
of biology. 

This text has been prepared in response to a real and widely recognized 
need. The experience of the authors in teaching the principles of breed- 
ing to undergraduate students has forced home the conviction that an 
adequate presentation in a single text of the facts and principles of 
genetics and their practical applications is a prime necessity. Those 
familiar with the literature of the subject will appreciate the magnitude 
of the task and, we trust, will be lenient in criticizing our choice of 
subject matter. It is impossible to include many things of mutual 
interest to genetics and agriculture if the work be limited to a single 
volume. We are keenly aware of many deficiencies and it is our desire 
to prepare a revised edition of the book in the near future. With this 
in view the suggestions of others are earnestly solicited. 

We take this opportunity to express gratitude to all who have rendered 
assistance, especially to those who have read portions of the manu- 
script or assisted in proof-reading and to all who donated or loaned photo- 
graphs or who assisted othei'wise with the illustrations. The onus of 
the work has been lessened in no small degree by the interest and en- 
couragement of our colleagues. The Authors. 

Berkeley, California, 
Feb. 18, 1918. 



CONTENTS 

PART I 
Fundamentals 
CHAPTER I 

The Methods and Scope of Genetics 

Page 

Introductory 1 

Genetics defined 1 

Content of genetics 1 

Variation and Heredity defined 3 

The problems of genetics 4 

The methods of genetics 4 

Prerequisites for genetics 10 

The application of genetics 11 

Genetics in agriculture 12 

CHAPTER II 

Variation 

Introductory 14 

Darwin and variation 14 

The universality of variation 14 

The variation concept 15 

Classification of variations 15 

Variation and development 20 

Variation and environment 21 

CHAPTER III 

The Statistical Study of Variation 

Introductory 32 

Causes of fluctuations 32 

Law of statistical regularity 32 

Law of deviations from the average 34 

The normal curve and its significance 34 

Requirements of biometrical study 36 

Biometrical terms 37 

Requisites to reliability 37 

Grouping variates — frequency table 38 

Frequency graphs 39 

The mean, standard deviation and coefficient of variability 41 

Theory of error 44 

ix 



X CONTENTS 

Page 

Multimodal curves 48 

Correlation and the correlation coefficient 49 

Regression 55 

Employment and value of the statistical method 56 

CHAPTER IV 

The Physical Basis op Heredity 

Introductory 57 

Heredity a consequence of the genetic continuity of cells 57 

The chromosomes 57 

Somatic cell division 59 

The production of germ cells GO 

Synapsis — its significance 63 

Independent distribution of chromosomes 63 

Number of chromosome combinations 65 

Chromosomes and sex in Drosophila 65 

Recapitulation of the mechanism of heredity 67 

CHAPTER V 

Independent Mendelian Inheritance 

Introductory 68 

Mendelism essentially statistical 68 

Mendel and his discovery 68 

The monohybrid 68 

Mendelian terminology 71 

The chromosome interpretation in a monohybrid 73 

Sex-linked inheritance 74 

The mathematical adequacy of Mendelism 77 

The dihybrid 81 

A case in maize 81 

The chromosome interpretation 33 

Mathematical consideration 83 

A case in guinea pigs 85 

A case in Drosophila 87 

The trihybrid 90 

A case in snapdragons 90 

Multi-factor hybrids 94 

Four-fold factor segregation in mice 94 

Methods of dealing with genetic data 95 

Validity of segregation ratios 100 

CHAPTER VI 

Linkage Relations in Mendelism 

Introductory 104 

Chromosome numbers and factors 104 



CONTENTS xi 

Page 

A case of partial linkage in maize 105 

Chromosome interpretation of linkage 109 

Linkage in Drosophila 110 

The factor groups 110 

An illustrative case from the first group 113 

Linkage in the second and third groups 113 

Linear arrangement of factors 115 

Application in Drosophila 116 

The mode of inheritance in crossing-over 121 

Experimental verifiratiou of theory of linear arrangement of factors 121 

Interference 122 

Linkage phenomena in other plants and animals 125 

Mathematical relations in linkage of phenomena 127 

CHAPTER VII 

Thk Nature and Expressiox of Mendelian Factors 

Introductory 129 

Conception of factors as loci in chromosomes 129 

Factors the genetic representatives of characters 129 

Manifold etfects of factors 133 

The variability of factor expressions 134 

Duplicate factors 136 

Multiple factors 137 

CHAPTER VIII 

Allelomorphic Relationships in Mendelism 

Introductory 144 

Character relationships in Pisum 144 

Dominance defined 144 

The extent of dominance 144 

Intermediate expression in the hybrid 147 

Variable character expression in the hybrid 149 

Competitive action of factors 150 

Mosaic expression of a character 152 

The presence and absence hy])othesis 153 

Multiple allelomorphism 155 

In Drosophila 156 

In other species 158 

Arguments in favor of the conception of multiple alleloinorijliism 160 

CHAPTER IX 

Types of Factor Interactions 

Introductory 163 

Aleurone color factors in maize 163 

Comb characters in fowl lt)4 



xii CONTENTS 

Page 

A factor system in. stocks 166 

Truncate winged Drosophila 168 

The factor explanation of reversion 170 

Darwin's hybridization experiments with pigeons 171 

Factor analysis of plumage color in pigeons 172 

CHAPTER X 

Factor Relations in Quantitative Inheritance 

Introductory 174 

Meaning of quantitative inheritance 174 

Tall and dwarf races 174 

Bush and cupid sweet peas 175 

Other factor differences affecting size 177 

The cotton leaf factor 179 

Corolla length in tobacco 181 

Mathematical requirements of the multiple factor theory of size inheritance . . . 185 

Castle's hooded rats 186 

Hypothesis of factor variability 192 

Arguments for the multiple factor theory . 194 

CHAPTER XI 

Inheritance of Sex and Related Phenomena 

Introductory 196 

The Drosophila or XY type of sex-inheritance 196 

Sex-linked inheritance 197 

Non-disjunction in Drosophila 198 

Secondary non-disjunction 201 

Non-disjunction not due to a sex-linked factor 204 

Bearing of non-disjunction on chromosome constitution of sex 204 

The WZ type of sex-inheritance 205 

Analogy between the two types 209 

Sex-determination in certain insects 210 

Sex-determination in certain plants 212 

Secondary sexual characters 214 

The question of sex factors 215 

Intersexual forms in Lymantria 216 

Analogous case in tobacco 218 

Conclusion 218 

CHAPTER XII 

Species Hybridization 

Introductory • 219 

Genetic compared with taxonomic differences 219 

Species hybrids in Antirrhinum 220 

Species hybrids in Cavia 223 



CONTENTS xiii 

Page 

The forms of species hybrids 227 

The vigor of species hybrids 230 

SteriUty in species hybrids 234 

Partially sterile hybrids of wheat and rj-e 236 

Partially sterile hybrids in Nicotiana 238 

Species hybridization in ffinothera 244 

Conclusions 248 

CHAPTER XIII 

Puke Lines 

Introductory 250 

Discovery of pure lines 250 

Conditions necessary for the existence of pure lines 255 

Isolation of pure lines from mixed populations 256 

The effect of selection within pure lines ... 257 

Significance of the pure line principle in breeding 259 

CHAPTER XIV 

. Mutations 

Introductory 263 

Two classes of mutations 263 

Chromosome aberrations 263 

Factor mutations 263 

The nature and causes of factor mutations 266 

Factor mutations, both germinal and somatic 268 

Vegetative mutations versus somatic segregation 272 

" Mutations " in the evening primroses 276 

PART II 
Plant Breeding 

CHAPTER XV 
Introduction, Historical 

Introductory • 287 

The beginnings of plant breeding 287 

Pioneers in plant breeding 288 

More recent progress in plant breeding; classification of methods 291 

Mass selection 291 

Line selection 293 

Hybridization 294 

Clonal selection 298 

Organization of plant breeding work 299 

Seed and plant introduction 299 

Collections of plant breeding material 299 

Research on plant groups 300 



xiv CONTENTS 

Page 

Organizations of plant breeders 300 

Summary 301 

CHAPTER XVI 

On Varieties in Plants 

Introductory 302 

Extent of variety differentiation in plants 302 

The origin of domestic varieties of plants 302 

Origin of sweet pea varieties 303 

Flower color in sweet peas 304 

Form and size in sweet peas 304 

Habit in sweet peas 308 

Hybridization and selection in the sweet pea 309 

Creation of varieties of the rose 310 

Origin of varieties in the Boston Fern 312 

CHAPTER XVII 

The Composition of Plant Populations 

Introductory 317 

Reproduction in plants 317 

Plants normally self -fertilized 317 

Plants normally cross-fertilized 317 

Discussion 318 

Populations of plants normally self -fertilized 320 

Populations as affected by crossing 321 

Summary 324 

CHAPTER XVIII 

Selection 

Introductory 325 

Selection methods in maize breeding 325 

Inbreeding in maize 325 

The ear-to-row method 327 

The Ilhnois Station experiments 327 

The remnant system 335 

Selection methods in close-pollinated plants 336 

The plant-to-row method 337 

Ineffectiveness of continued selection within pure lines 339 

The practical importance of keeping varieties pure 341 

CHAPTER XIX 

Hybridization 

Introductory 342 



CONTENTS XV 

Page 

Purpose and plan 342 

General method 343 

Method of hybridiziuf? maize 344 

Method of hybridizing wheat 345 

Method of hybridizinjr alfalfa 348 

Some of the difficulties attending hybridization 350 

Conditions favorable for hybridization 351 

Species hybridization 351 

Svalof method of creating populations 352 

CHAPTER XX 

Utilization of H-i'BRiDs in Plant Breeding 

Introductory 353 

Purposes of hybridization 353 

Increased production in Ft maize hj'brids 353 

Crossing inbred strains or biotypes 354 

Effect of inbreeding in strains of maize 355 

Method of comparing yields 356 

Crossing species, subspecies, varieties and strains 358 

Superior qualities of first generation hybrids 359 

Immediate effect of crossing on size of kernel 360 

Centralized seed corn production 361 

A method of producing hybrid corn seed 361 

Application in other annual crop plants 362 

Application in vegetatively propagated plants 364 

Rapid growing tinker and ornamental trees 365 

CHAPTER XXI 

Mutations in Plant Breeding 

Introductory 366 

Occurrence of mutations 366 

Mutations in crop plants 367 

The search for mutations 370 

Propagation of mutations 372 

CHAPTER XXII 

Graft-hybrids and Other Chimeras 

Introductory 374 

Definition of graf t-hybritl 374 

Winkler's tomato-nightshade graft-hybrids 374 

Baur's investigation of a natural chimera 378 

Other natural chimeras ' 381 

Two categories of variegation 381 

The physiological behavior of graft hybrids 382 

Modification of one graft-symbiont by the other 383 



xvi CONTENTS 

CHAPTER XXIII 
Bud Selection 

Page 

Introductory 385 

Efficacy and practicability of bud selection 385 

Bud variation in plants 385 

Bud selection in Coleus 386 

Bud selection in horticultural practice 391 

Performance records as a basis for bud selection 391 

Bud mutations in Citrus •. . . . 392 

Deciduous tree fruits 393 

"Pedigreed" nursery stock 394 

Bud selection in the potato 394 

Certified seed potatoes 397 

Other crops in which bud selection may apply 398 

Limitations of bud selection 399 

CHAPTER XXIV 

Breeding Disease Resistant Plants 

Introductory 400 

The causes of plant diseases 400 

The nature of disease resistance in plants 401 

Disease resistance in natural species 401 

Phylloxera-resistant grapes 402 

Endothia-resistant chestnuts 405 

Blight-resistant pears ^ 407 

Breeding disease resistant varieties by hybridization 408 

Creating rust-resistant wheats 411 

Inheritance of disease resistance in other plants 413 

Breeding disease resistant plants by selection 416 

CHAPTER XXV 

Plant Breeding Methods 

Introductory. , 419 

Need of systematic methods 419 

Pedigree culture 419 

The Svalof system 425 

Variety tests — purposes; difficulties involved 427 

EstabUshing varietal types 427 

Determining best varieties for given locations 428 

Strain tests — purpose; difficulties involved 432 

Plant-to-row tests 433 

Factors that affect experimental results 433 

CHAPTER. XXVI 

General Considerations and Conclusions 

Introductory 437 



CONTENTS xvii 

Page 

The relation of science to plant breeding — historical review 437 

The future relation of genetics and plant breeding 440 

Planning breeding operations in the light of scientific knowledge 440 

PART III 
Animal Breeding 

CHAPTER XXVII 

The General Aspects of Animal Breeding 

Introductory 443 

The history of animal breeding 443 

The animal breeding industry 445 

The art of breeding 447 

The problems of animal breeding 447 

The service of genetics 448 

The service of genetics in education 450 

The personal equipment of the animal breeder 450 

CHAPTER XXVIII 

Variation in Domestic Animals 

Introductoiy 453 

The sources of variation 453 

Selection as a cause of variation 454 

Variation by modifiability 454 

Modifiability and breeding value 456 

Modifiability and correlation 459 

Variation by recombination 459 

Mutation in domestic animals 462 

CHAPTER XXIX 

Mendelism in Domestic Animals 

Introductory 465 

Importance of experimental breeding 465 

Mendelism in horses 465 

Mendelism in sheep 475 

Mendelism in swine 476 

Mendelism in poultry 476 

CHAPTER XXX 

Acquired Characters in Animal Breeding 

Introductory 480 

The problem 480 

The beUef in the inheritance of acquired characters 482 

The argument against the inheritance of acquired characters 484 



xviii CONTENTS 

Page 

The soma and germ plasm — experimental investigations .... 485 

The isolation of the germ plasm 488 

The inadequacy of affirmative evidence 489 

The transmission of functional modifications 491 

Parallel induction 492 

The adequacy of other factors 493 

The conclusion 494 

CHAPTER XXXI 

* The Selection Problem in Animal Breeding 

Introductory 495 

General views of selection 495 

The American standard bred horse 495 

Fecundity in fowls 497 

Bantam fowls 499 

Selection and breeding methods 500 

Selection indices 502 

CHAPTER XXXII 

Hybridization in Animal Breeding 

Introductory 508 

Growing importance of hybridization 508'' 

Grading 508 

Crossbreeding • 514 

Species hybridization among domestic animals 515 

CHAPTER XXXIII 

Disease and Related Phenomena in Animal Breeding 

Introductory 522 

The inheritance of disease 522 

The inheritance of predisposition to disease 523 

The inheritance of defects 524 

Defects in domestic animals 526 

Immunity to disease 527 

Breeding for immunity 529 

CHAPTER XXXIV 

Sex in Animals 

Introductory 536 

The determination of sex 536 

Sex-determination in mammals 536 

Sex-determination in birds 539 

The sex-ratio 539 



CONTENTS xix 

Page 

Causes of unusual sex-ratios 542 

Metabolic theories of sex-determination 544 

Inheritance of unusual sex-ratios 546 

Secondary sexual characters 548 

The effects of castration 548 

CHAPTER XXXV 

Fertility in Animals 

Introductory 551 

Factors influencing fertility 551 

The Darwinian theory of fertility 552 

Inbreeding not in itself harmful 553 

Fertility as related to Mendelian factors 554 

The chromosomes and fertility — Drosophila 555 

Sterility in other animals . . 555 

Sterility in hj^brids 556 

FertiUty as related to heterozygosis 559 

Fecundity in fowls 559 

Conclusion 563 

CHAPTER XXXVI 

Some Beliefs op Practical Breeders 

Introductory 564 

Telegony 564 

Harmful effects of hybridization 571 

Infection of the male. Saturation 571 

Maternal impression 572 

Prepotency 573 

The Mendelian interpretation 574 

The relative factor interpretation 575 

The hereditarj^ complex interpretation 575 

Greater prepotency in the male 576 

Conclusions 576 

CHAPTER XXXVII 

Methods of Breeding 

Introductory 577 

Methods means to an end 577 

Phenotypic selection 577 

Limitations of phenotypic selection . 578 

Pedigree breeding 580 

Breeding systems based on blood relationship 580 

Inbreeding 581 

Line breeding 582 

Out-breeding 583 

Other systems of breeding 583 

Genotypic selection 584 



XX CONTENTS 

CHAPTER XXXVIII 
Methods of Conducting Breeding Investigations 

Page 

Introductory 591 

The need of records 591 

Judging the individual 591 

Pedigrees " . . 596 

The coefficient of inbreeding 598 

The coefficient of relationship 600 

Marking individuals 602 

Recording data 603 

Cooperative breeding 606 

CHAPTER XXXIX 

Concluding Remarks 

Introductory 607 

The present lack of detailed knowledge 607 

The need of research 608 

The service of genetics 609 

The need of other knowledge 611 

Glossary 615 

List of Literature Cited 622 

Index 648 



GENETICS 



IN 



RELATION TO AGRICULTURE 



PART I-FUNDAMENTALS 

CHAPTER I 
THE METHODS AND SCOPE OF GENETICS 

Soon after Mendel's report of investigations in heredity had been 
rediscovered, it became evident to most biological investigators that a 
flood of light had been thrown upon the problem of heredity, and the 
related subjects of variation, development, and evolution. The need 
for a new term, therefore, to designate this interrelated portion of bio- 
logical science led Bateson to coin the word, genetics, from the Greek 
root, TEN, "become." The derivation does not indicate, it must be 
admitted, very clearly the portion of biology to which the term genetics 
applies, but this vagueness has in it an element of desirability, for it is 
extremely difficult to define accurately the boundaries which delimit the 
province of genetics. Bateson himself has stated that genetics deals with 
the physiology of heredity and variation; and a favorite statement of 
authors has been that genetics is the science of the origin of individuals. 
But these statements — they can hardly be called definitions — must be 
qualified carefully in order that they may be understood. Accordingly 
it has seemed desirable to construct a definition of genetics in purely 
objective terms. The following definition is, therefore, proposed to ful- 
fill this need; it, too, will require some qualification: 

Genetics is the science which seeks to account for the resemblances 
and the differences which are exhibited among organisms related by 
descent. 

The Content of Genetics. — If genetics be defined in the above manner, 
it may be stated roughly that variation is that portion of genetics 
having to do with the differences beween organisms, whereas heredity has 
to do with the resemblances which they exhibit. But this statement 
does not define very accurately the exact meanings of the two terms; to 
do this it is necessary to consider certain fundamental facts. 

Organisms exhibit various degrees of difference and resemblance, and 
classification is made possible first, by resemblances between individuals 
and, second, by differences between groups of individuals. Further, the 
orderly interrelations which are exhibited by living beings in general has 

1 



2 GENETICS IN RELATION TO AGRICULTURE 

made it possible to group them into orders, families, genera, and species 
according to the degree of resemblance which exists among groups of 
individuals. But this is merely a view en masse of the differences be- 
tween organisms, for it is universally true that no two individuals are 
exactly alike. There are, therefore, for all practical purposes, two orders 
of difference between individuals; first, racial differences, those which 
separate groups of individuals, and second, individual differences, those 
which distinguish the individuals of a group from one another. Strictly, 
of course, there are all possible gradations from the one degree of dif- 
ference to the other, but conveniently it may be said that the former, 
the racial differences, are those which characterize different lines of 
descent, whereas the latter, the individual differences, distinguish indi- 
viduals within a given line of descent. The problem as to the origin of 
racial differences is a problem of evolution; the problem of the origin of 
individual differences is a problem of genetics, and we accordingly shall 
construct our definition of variation to apply to differences exhibited 
by individuals related by descent. 

Now all multicellular organisms which reproduce by sex exhibit the 
common characteristic of two distinct cycles of cellular development; 
gametogenesis, or development of the germ cells, and somatogenesis or 
development of the body. The resemblances which make it possible 
to group individuals into orders, famiUes, genera, and species are the 
result of the fundamental relation which exists between these two 
cycles, for it is a commonplace fact that the germ cells of any species 
can reproduce individuals of the same and no other species. This rela- 
tion of germinal constitution to the development of the soma is specific 
for all classes and grades of characters, but the order of specificity may 
be either racial or individual, just as the order of difference between 
individuals is racial or individual. 

The term variation carries with it the idea of deviation from type, 
and obviously the above statements, brief as they are, of the cycles in 
individual development leave room for several possibilities of deviation 
from type. Thus, if we look at the matter from one point of view, the 
guiding hand in determining the characters of the individual is the 
specificity of the germinal substance. But every individual develops 
under a certain set of conditions, the environment, which is independent 
of the germinal substance; and these conditions have a certain, usually 
merely modifying, influence in the development of the individual. 
There is, therefore, a possibility for differences to arise in individuals 
independently of differences in the germinal substance, differences which 
are specifically attributable to diversities in the environment, and 
which may have no effect on the germinal substance itself, just as the 
degree of heat, for example, may cause a variation in the end products 



THE METHODS AND SCOPE OF GENETICS 3 

which a given chemical system yields. Differences in development may, 
also, occur because of actual diversities in the germinal substance, and 
these may arise from the intermingling of different kinds of germinal 
substance, such as obviously takes place in sexual reproduction, a 
cause of variation which has been ably advocated by Weismann and 
styled by him amphimixis; or they may arise from actual changes 
in the germinal substance, distinct from the intermingling of germinal 
elements which already exist, a form of variation which has been 
proposed and elaborated by de Vries under the name of the mutation 
theory. Accordingly the term variation in genetics is so defined that it 
includes differences in individuals related by descent, although many 
authors do not include within the term those differences which are due to 
environmental conditions of all categories. The following definition is 
framed in conformity to that already given for genetics. 

Variation is difference, whether in the expression of somatic characters 
or in the elements of germinal substance, among organisms related by 
descent. 

Heredity is commonly defined as the tendency of offspring to develop 
characters like unto those of their parents; according to Castle it is 
resemblance based upon descent. Thomson presents a very able dis- 
cussion of the concept, heredity, together with criticism of definitions 
which have been offered from time to time for the term. According to 
his definition, by heredity is meant nothing more nor less than organic 
or genetic relation between successive generations. 

The universal tendency of organisms to produce similar organisms 
is the cause of the maintenance of organic groups and group relations. 
But experimental research has demonstrated that sometimes new com- 
binations of germinal substance produce characters which have not been 
exhibited by parents. It is necessary, therefore, to define heredity in 
such general terms that it will include those exceptional characters which 
have never been exhibited by any ancestor. Now regardless of any 
external difference which may be exliibited by an individual, its germinal 
constitution bears a perfectly definite relation to those of its parents. 
For that reason the following definition is stated in terms of elements of 
the germinal substance, rather than in terms of somatic characters. 

Heredity is germinal resemblance among organisms related by 
descent. 

Finally, with respect to the content of genetics, emphasis should 
be laid upon the importance of a consideration of the various phases of 
development. In development are included all those changes and cycles 
through which the individual passes in attaining the adult condition. 
Obviously there is much in development which cannot be treated at all 
in an elementary text-book of genetics, for particular cycles or phases of 



4 GENETICS IN RELATION TO AGRICULTURE 

development are treated as separate sub-divisions of biology, such as 
embryology, cytology, experimental morphology, and like subjects. 
While obviously there is much in all of these subjects which is irrele- 
vant to a treatment of genetics, nevertheless, rightly interpreted, there 
is little which is essential to any one of them which does not bear some 
more or less intimate relations to those phenomena which belong more 
strictly in the province of genetics. The reason for this is very apparent, 
the development of the individual is a consequence of the elaboration 
of the hereditary material, it is the fulfillment of the possibilities wrapped 
up in the germ cell; how then can it fail to possess much that is of very 
great significance to genetics? Assuredly the further advancement of 
the science of genetics will focus more and more attention upon the prob- 
lems of growth and differentiation in the individual ; for that reason these 
emphatic statements are made. 

The Problems of Genetics. — Obviously the problems of genetics 
are those which grow out of a study of resemblances and differences in 
individuals related by descent. Wilson has reduced the statement of 
the problems of inheritance and development to that oft-quoted question : 
"How do the adult characteristics lie latent in the egg; and how do 
they become patent as development proceeds?" Pearl has voiced very 
much the same thought in his statement that the critical problem of 
inheritance is the problem of the cause; the material basis; and the 
maintenance of the somatogenic specificity of germinal substance. 

There are four general methods of attacking the problems of heredity; 
namely, the methods of observation, experimental breeding, cytology, 
and experimental morphology. Each of these methods has its specific 
advantage and particular value as well as its definite limitations. In the 
following discussion each method is considered briefly with respect to its 
relation to the development of the science of genetics. 

The Method of Observation. — The method of observation, or de- 
scription as it is often called, requires special treatment because it employs 
the inductive mode of reasoning. Briefly the essential steps involved 
in the application of inductive reasoning to the problems of genetics 
may be stated as follows. The first step is the observation of the re- 
semblances and difl"erences between representative individuals of a 
given line of descent or, if problems of evolution are under consideration, 
of different lines of descent. The next step is a comparison of the ob- 
servations which have been made for the purpose of determining whether 
they show orderliness with respect to each other; in other words to de- 
termine whether they probably have a common causal basis. If they 
do show such orderliness, an attempt is made to formulate the principles 
or laws which govern them. Finally, the principles or laws thus for- 
mulated are applied to other instances not included in the original set of 



THE METHODS AND SCOPE OF (lENETlCS 5 

observations in order to test their general validity. The weakness of 
the method in biology lies in the lack of rigid experimental control over 
the phenomena which are under observation, and also in the fact that 
often it is either very difficult or impossible to subject to experimental 
verification the principles or laws which have been thus formulated. For 
this reason, the method of observation as a means of formulating prin- 
ciples and laws must constantly be subjected to rigid scrutiny, lest unde- 
tected fallacies lead to the acceptance of conclusions which actually have 
no significance from a biological standpoint. 

But although the observational method has very definite limitations in 
the determination of genetic principles, nevertheless it has been the chief 
method of investigation in the formulation of some of the most stimu- 
lating theories of biological science. The marshalling of evidence by 
Darwin in support of the evolution theory depended almost entirely on an 
application of this mode of research to a vast array of more or less iso- 
lated cases. The mass of evidence, which he accumulated in order to 
demonstrate that natural selection by favoring the "survival of the 
fittest," to use Spencer's phrase, results in evolutionary progress in suc- 
ceeding generations, .will ever stand as a monument to his masterly skill 
in observation and interpretation. 

In addition to its utilization in the development of the evolution 
theory, the observational method has been employed widely in the field 
more strictly included in genetics. Sir Francis Galton employed a 
refined type of the observational method in his study of heredity. His 
object was to establish a law of organic resemblance within a single 
species, distinctly a problem of genetics. In order to do this he employed 
a system of more exact observation based upon accurate determinations 
in a large number of instances and mathematical reduction of the data 
thus collected. This system has since undergone notable development, 
particularly at the hands of Karl Pearson, and, as biometry, it is often 
accorded recognition as a distinct branch of biology. As one of the re- 
sults of his studies, Galton announced the law of ancestral inheritance 
which states that on an average each parent contributes one-quarter or 
0.52, each grandparent one-sixteenth or 0.5*, and so on to the total 
heritage of the individual, which equals 1.0. The other notable result 
of these studies, the law of filial regression states essentially that on the 
average any deviation from racial type is transmitted to the offspring in a 
lessened degree, so that;, in general, offspring differ less from the type 
of the race than their parents; specifically they exhibit a deviation from 
the racial mean only two-thirds as great as the parents. 

Mere observation, be it ever so precise, is subject to very decided 
limitations when employed as a method of analyzing the general problems 
of evolution and heredity. To be convinced of this, one need only con- 



6 GENETICS IN RELATION TO AGRICULTURE 

sider the opinions which have been entertained by those who have em- 
ployed this method in the solution of biological problems. Thus Darwin 
believed that minute continuous variations are transmitted and form 
a basis for evolution and that the more striking discontinuous var- 
iations are of little moment in the origin of species. These are beliefs 
which rigid experimental investigation has failed to establish, and which 
are, therefore, highly improbable. In fact it has been clearly demon- 
strated that minute differences between individuals are for the most 
part not transmitted, and that distinct new characters which appear 
suddenly are often heritable. Similarly, the inheritance of acquired 
characters, so readily accepted by men with minds as keen as those of 
Darwin and Spencer, has failed to receive confirmation when subjected 
to rigid experimental enquiry. Definite knowledge on points such as 
these is of tremendous importance in making for progress toward the 
solution of the general problems of genetics, but such progress is slow 
and uncertain by the employment of the observational method of attack 
alone. It is for this reason that the favor of geneticists has swung so 
strongly toward a more rigid method of experimentation. 

However, the observational method is not unique in possessing limita- 
tions. No single method is known invariably to give correct results. It is 
necessary to combine all available methods in order to insure the most 
certain and rapid approximation to the truth. But the difficulty with 
the observational method, particularly that part of it known as biometry, 
has been in the manner of its employment in the elucidation of genetic 
phenomena. It has been employed, as Pearl points out, both as a method 
of research and as a method of stating the results of experience. The 
former manner of utilization is unquestionably of great value in genetic 
research, its particular value residing in the fact that it has substituted 
exact methods of expression for vague and indefinite statements. It has 
performed a service of tremendous value to biology in the introduction 
of the probable error concept as an index of the degree of reliance to be 
placed in the results of determinations arrived at by other methods. 
The latter manner of utilization, however, as a method of stating the 
results of experience, the employment of which is characteristic of the 
biometrical school, is subject to serious objections. However, it is worthy 
of note that the method of observation will ever remain a valuable aid 
to the extension of knowledge, particularly in directions in which, by 
their very nature, it is impossible to employ experimental methods of 
research. It is difficult to imagine, for instance, any notable advance 
in our knowledge of human heredity save by a proper employment of 
this method of investigation. 

The Method of Experimental Breeding.— The essential feature of all 
experimental breeding is the raising of pedigreed cultures of plants and 



THE METHODS AND SCOPE OF GENETICS 7 

animals, for which reason it is sometimes called the pedigree method. 
The notable progress which has been made in genetics during the past 
few decades has come from the application of this mode of enquiry. It 
is the analytic method of the geneticist and it is often and not unjustly 
compared, both with respect to its utility and its limitations, to the test- 
tube method of analytical chemistry. From it have come many stimu- 
lating ideas of heredity and variation; the Mendelian theory of heredity; 
the closely related pure line theory of Johannsen; and the mutation 
theory of de Vries: few methods of research can boast a more honorable 
array of achievements. 

Of these achievements, the Mendelian theory is the accepted founda- 
tion of present ideas of heredity. For the application of Mendelian methods 
of analysis three essential breeding operations are necessary ; first,the raising 
of pedigreed strains of plants and animals to determine their behavior 
under controlled conditions; second, the hybridization of diverse races; 
and third, the intensive study of the hybrid progeny through successive 
generations. From this outline of the breeding methods which are 
employed, it may be concluded rightly that the Mendelian method, like 
the Galtonian, is essentially statistical. It differs radically, however, 
from the Galtonian method in that it substitutes the observation of con- 
trolled progenies for that of ancestral generations. Its particular ad- 
vantage lies in the fact that it is strictly verifiable. Moreover, it has had a 
different and more specific purpose in view, namely to state in definite 
terms how the particular individual will behave in heredity, rather than 
to arrive at a determination of average behavior in this respect. The 
important result of this method of analysis has been to demonstrate that 
the germinal material is made up of definite units or factors which stand 
in close relationship to particular characters of the soma, and to demon- 
strate how these elements of the germinal substance are transmitted from 
generation to generation. 

The two remaining products of the pedigree culture method, the pure 
line theory and the mutation theory, stand in close relationship to the 
Mendelian theory of heredity; because they may be interpreted in terms 
of the elements which constitute the germinal substance. Of these the 
pure line theory may be said to add another conception to those of the 
Mendelian theory, namely that elements of the germinal substance 
possess a high degree of stability. If this conception be accepted, it 
follows — and this is the central postulate of the pure line theory — that 
variation among individuals of like germinal constitution is a response 
to external or internal conditions which are not reflected in the germinal 
substance. Such variations, therefore, are of no consequence for the 
establishment of new hereditary characters. A large number of plants, 
among them barley, oats, rice, wheat, and practically all the legumes, 



8 GENETICS IN RELATION TO AGRICULTURE 

are almost invariably self-fertilized. They consequently give rise auto- 
matically to populations which are composed entirely of pure lines. 
The pure line theory, therefore, has tremendous practical significance. 

The mutation theory adds yet another conception to those which have 
already been stated, namely that of occasional mutability of germinal 
elements. It is, therefore, directly contradictory to the pure line theory 
in its fundamental postulate; but the very great infrequency with which 
changes occur in germinal elements saves the pure line theory from 
inutility. Here the important result has been to establish firmly the 
occurrence of occasional, definite, discontinuous changes in germinal 
substance in consequence of which new characters are added to the 
heritage of the race. Much of the variability in individual characters 
which is exhibited by plants and animals appears to have had its begin- 
ning in mutational changes in the germinal substance. The mutation 
theory, therefore, is another consequence of genetic investigations which 
has far-reaching practical consequences. 

Fruitful as have been the results of the method of experimental breed- 
ing in prosecuting genetic research, students and investigators should 
not delude themselves as to the nature of the knowledge which it has 
yielded. It cannot stand alone as a mode of investigation, for even the 
present illuminating conception as to the structure and operation of the 
hereditary mechanism has been almost as much the result of cytological 
as of breeding investigation. But taking this conception in its present 
form, tremendous as has been the advance of recent years, this sort of 
knowledge cannot represent the ultimate goal of genetic research. 
Mendelism has given us the plan of heredity^the more intimate and 
fundamental knowledge of the material which is employed in the elabora- 
tion of that plan remain the task of some other mode of research. 

The Method of Cytology. — The method of cytology in genetic re- 
search is concerned primarily with questions of cell mechanism. It may 
be said to be directed toward the solution of two distinct problems, first 
the behavior of the hereditary elements in somatogenesis, the building 
up of the body, and secondly in the determination of the nature and 
operation of the mechanism which distributes hereditary elements from 
parent to offspring. These are matters of fundamental importance in 
genetic enquiry; it is unfortunate that the methods of dealing with the 
problems here presented are necessarily static and so little under the 
control of the investigator. Nevertheless even with these handicaps, the 
contributions of cytology to genetic interpretation are by no means 
inconsiderable. The determination of the equivalent distribution of the 
hereditary elements in the cell divisions of somatogenesis and the prob- 
able fact that every ultimate cell in the body normally possesses all the 
hereditary elements of the initial fertilized egg-cell have been established 



THE METHODS AND SCOPE OF GENETICS 9 

as nearly as may be by cytological research. Moreover, the separation 
of homologous contributions of the parents in the formation of germ cells 
and the union of two homologous sets of hereditary elements for the 
production of new individuals represent another phase of the problems 
which have been solved by cytological research. 

Although obviously the dangers of misinterpretation in dealing with 
fixed and stained preparations of cells or sections of cells are very great, 
a fact which is disclosed by the diverse interpretations which different 
investigators have given of the same phenomena and structures, never- 
theless the importance of this field of research should not be under- 
estimated on that account. There are several reasons for reposing confi- 
dence in the results of cell investigations, and these come from two 
sources; from the confirmations of the growing field of what may be 
called experimental cytology, the observation of cell phenomena directly 
in living cells, and from the broad general result of cytological research 
that the mechanism which has been discovered is by nature such an one 
as might be expected from a yriori consideration of the results of 
Mendelian investigations. The close correspondence which exists 
between cell behavior as it is believed to exist from cell investigations 
and hereditary phenomena as they are known to exist from Mendelian 
investigations has given renewed confidence to students of heredity in the 
validity of their interpretive conclusions. 

The most important progress which has been made within the last 
decade in genetic science has been that of interpreting Mendelian phe- 
nomena of inheritance in terms of the behavior of the cell mechanism. 
Thus far this work has been carried to any degree of completeness in 
only one species, the common fruitfly, Drosophila ampelophila. In the 
extensive investigations which have been made with this species, Morgan 
and his associates have demonstrated how close a correlation exists all 
along the line between cell behavior and hereditary distribution of 
characters. Certain characters -are distributed independently of each 
other, the pairs of chromosomes separate independently of each other in 
the formation of gametes; certain characters display irregularities in 
distribution and expression associated with differences in sex, the chromo- 
some content of the two sexes is demonstrably different; four sets of 
characters exist the members of which tend to remain together in trans- 
mission in the combinations in which they occurred originally , the entire 
chromatin material is contained in four pairs of chromosomes; and 
finally irregularities in character distribution have been discovered, the 
chromosome constitution and distribution in such cases are correspond- 
ingly irregular. These facts the student will be better fitted to appreciate 
later on; they are given here to show how the results of one method of 
investigation are supported and strengthened by those of other methods. 



10 GENETICS IN RELATION TO AGRICULTURE 

The Method of Experimental Morphology. — Under the heading 
morphology, we include those particular phases of development which 
are designated by the terms, ontogeny and embryology. The method 
of experimental morphology has for its task the solution of the problem 
of the development of the individual as it is related to problems of 
variation and heredity. The aim of this method is to determine how 
the characters of the adult become patent as development proceeds, the 
broad question of the origin of complexities within organisms. 

In the Mendelian method, the formal relations which exist between 
hereditary elements are dealt with, particularly their relations in dis- 
tribution and recombination. The characters of the adult organism are 
for the most part the basis of judgment. In spite of the general truth of 
this statement, however, Mendelian analysis has in many cases extended 
into the field of the physiological relations which exist between hereditary 
elements, not merely with regard to contrasted homologous hereditary 
determiners, but with regard to the physiological relations existing in 
development between entire sets of hereditary elements, and at times 
even between these and definite factors of environment. But for the 
most part the solution of such problems depends upon thorough experi- 
mental study of development in individuals of known genetic constitution. 
This portion of the problem remains almost untouched. If development 
be thought of as a series of successive physico-chemical reactions, the 
complexity of the problem may easily be judged. Certain of the simpler 
features of it, however, have been attacked and the results of these 
preliminary studies have indicated still other modes of approach, so that 
we may expect that when geneticists come to appreciate the light which 
may be thrown upon heredity by the experimental investigation of 
development, research in this field will be greatly stimulated. Already 
as Jennings has pointed out the main features of the process of develop- 
ment are clearly indicated; the hereditary elements of the chromosomes 
remain the same in each cell, the reactions and functions of any cell 
depend upon this chromatin system working in conjunction with the 
cytoplasmic matrix in which it is located. From this fact may be drawn 
the broad conclusion that differentiation within the individual depends 
upon cytoplasm differentiation. The difficulty of the question of how 
and why should not deter investigation. 

Prerequisites for Genetics. — The foregoing discussion of modes of 
research in genetics should indicate something as to the nature of the 
working equipment necessary for a study of the science. Since genetics 
is a biological science, intelligent study of it presupposes a thorough 
grounding in general biology such as is given in foundation courses in 
botany and in zoology. Inasmuch as practically all domesticated plants 
and animals belong to the higher orders, particular attention should be 



THE METHODS AND SCOPE OF GENETICS 11 

given to the cycles of developments in these organisms, especially those 
phases which are comprised in development and reproduction. Of 
particular importance is a general knowledge of physiology, not so much 
on account of the direct utility which it has in the study of genetics as 
for the attitude toward life phenomena which it awakens in the student. 
Genetics, indeed, is essentially a sub-division of physiology in the broader 
sense. A knowledge of mathematics is a valuable asset because it is 
often necessary to subject the data of heredity and variation to mathe- 
matical treatment in order to interpret them properly. For the elemen- 
tary study of genetics, a knowledge of the methods of dealing with simpler 
algebraic problems is sufficient; for advanced study a knowledge of the 
differential and integral calculus is highly advantageous. Finally it 
may not be out of place to mention the fact that investigation in genetics 
is not confined to those who employ the English language. A reading 
knowledge of French and German is practically necessary for those who 
desire to pursue the subject very far. 

The Applications of Genetics. — Genetics has both scientific and prac- 
tical applications. As an example of its scientific applications, the part 
which it has played in shaping doctrines of evolution instantly comes to 
mind, for of necessity such doctrines must conform to the fundamental 
principles of genetics. The science of genetics and that of evolution are 
by their very natures constantly encroaching each upon the fields of 
research of the other. Thus experimental investigations of evolution 
are of vital interest to genetics, because they deal with the mode of 
origin of hereditary characters. Genetics, also, has its applications in 
branches of biology other than that of evolution, indeed throughout the 
entire realm of biology its influence is felt in shaping thought and direct- 
ing interpretation. There are few other sciences which possess so much 
of general interest as that of genetics. 

The practical applications of genetics are found in agriculture and 
in human affairs. Here genetics involves many things which are extra- 
biological. Thus in agriculture emphasis is placed upon the employment 
of the principles of genetics for the amelioration of plants and animals for 
man's use. Breeding, then, may be defined as the art of improving 
plants and animals by hybridization and selection. To make effective 
progress along this line methods of testing given individuals or races, 
both with respect to fixity of type and comparative value, have been de- 
vised. The methods of attack are very much the same as those which 
are employed in the experimental study of heredity and evolution, the 
primary aim of which is merely to discover underlying principles. Eugen- 
ics is concerned with the principles of genetics in so far as they may 
be applied in the improvement of the human race; but it includes much 
that is sociological, rather than biological. The applications of the prin- 



12 GENETICS IN RELATION TO AGRICULTURE 

ciples of genetics, therefore, are always subject to such modifications as 
may be determined by practical considerations. 

Genetics in Agriculture. — Modern agriculturists, for the most part, 
appreciate fully the importance of producing only the best types of plants 
and animals; for in spite of the strange anomalies of economic conditions 
which at times appear to give actually a greater return for smaller total 
yields, the fact must remain that the larger view of the agriculturist's 
place in society requires of him as of all its other members the fullest 
possible returns compatible with economic principles and the require- 
ments for a permanent agriculture. But although the desirability of 
high production and quality is very generally recognized, it is a fact that 
very often this ideal cannot be attained except by the most careful and 
intelligent efforts. This is more often the case with plants than with 
animals, for plants are on the whole less independent of environmental 
conditions and therefore more susceptible to differences in them. Pro- 
ducers of crops are always in need of varieties which are better adapted 
to local conditions, but except in rare cases they are not fitted to develop 
such varieties. Here genetics comes very definitely to the aid of the 
plant breeder for its principles provide a safe guide for him in attaining 
his ideal. Already breeders of plants have realized a great saving of 
time and expense as a result of the application of principles derived 
from scientific investigations in their work. 

The animal breeder on the other hand has faced a somewhat different 
problem. The far greater comparative value of the individual in live- 
stock operations has led in animal breeding to the establishment of pure 
breeds of domesticated animals of remarkable excellence. Long applica- 
tion of the method of trial and error has developed a body of empirical 
knowledge which has achieved results nothing short of the marvelous. 
But while the old empirical methods have served their purposes well, 
nevertheless they cannot from their very nature give complete satisfac- 
tion. Knowledge is only secure when it rests upon a firm foundation of 
principle, and however excellent have been the results of empirical 
breeding from a utilitarian standpoint, they have not led to the discovery 
of fundamental principles. The principles of genetics provide a consist- 
ent interpretation of the results of breeding methods. To the novice a 
knowledge of such principles is an abundant aid in interpreting and organ- 
izing details of experience; by its help he can progress more safely and 
more surely in determining the methods of procedure which are es- 
sential to the fullest success in his breeding operations. The real service 
of genetics to animal breeding lies in the promotion of clarity of thought, 
and that is a thing of no little value. 

Although genetics thus far has contributed but little toward improve- 
ment of the existing methods of animal breeding, it is not a dream im- 



THE METHODS AND SCOPE OF GENETICS 13 

possible of realization that in the future its contributions in this direction 
will be of considerable importance. The science of genetics is still in 
its infancy, it is still in the formative period of its existence. It has not 
yet been possible with any degree of satisfaction to analyze the heredi- 
tary constitution of any farm animal, even to the incomplete extent 
which has been accomplished in some plants and in some of the smaller 
animals. Obviously we cannot apply even the general principles of 
genetics intelligently in animal breeding until we are more thoroughly 
conversant with the facts of character behavior and factor relationship. 
Such facts can only be determined by means of carefully planned experi- 
mental investigations. A few investigations have already resulted in 
important extensions of our knowledge in this respect, others now under 
way promise to extend this knowledge considerably further. Systematic 
crossbreeding of cattle and sheep for definite commercial purposes is 
of proven value. The method of breeding for high winter egg production 
in fowls has been determined. Investigation of the inheritance of high 
milk production in cattle is under way. Geneticists are also seeking 
to analyze the extensive data with respect to certain characters such as 
color, fecundity, and speed which have been recorded in herd books. 
Progress in such work with the larger domestic animals is necessarily ex- 
ceedingly slow, but this should not deter investigators from organizing 
carefully planned experiments to extend knowledge in this direction. It 
is only in this way that genetics can take its proper place in practical 
animal breeding. The progressive agriculturist can well afford to en- 
courage every proper effort having as its aim the collection of genetic 
data. 



CHAPTER II 
VARIATION 

Organic differences, their nature and causes, have furnished abundant 
material for speculative enquiry since time immemorial. The great sig- 
nificance of the fact of organic individuality was not fully grasped until 
Lamarck founded his theory of evolution which postulated the progressive, 
imperceptible change of one species into another. It remained for Darwin 
to scrutinize all phases of organic life, past and present, wild and domes- 
ticated, in his search for a guiding principle which should explain the 
course of evolution. Darwin's hypothesis of natural selection assumes 
variability without enquiring into its causes, but this does not mean 
that Darwin was not concerned with the problem of causes. In both 
his "Origin of Species" and "Variation in Animals and Plants under 
Domestication" the causes of variability are often referred to and he 
suggested among others, the kind and amount of food, climatic changes 
and hybridization. Our respect for the great naturalist's keen percep- 
tion deepens when we realize that very little has been added as yet to 
our knowledge of the causes of variation. 

The Universality of Variation. — Individuality is common to all or- 
ganisms. No two trees, no two leaves, no two cells in a leaf are identical 
in every respect. Individuals sometimes appear exactly alike but even 
identical twins will be found to differ in some features. The shepherd 
knows his sheep individually and the orchardist his trees. Were there 
no differences in individuals there would be no changes in species and 
there could be no improvement of cultivated plants. "Variation is at 
once the hope and despair of the breeder," the hope because without it 
no improvement would be possible, the despair because very often, when 
improvement has been made, variation results in a tendency to fall 
below the standard previously reached. In the sugar beet, for example, 
a high percentage of sugar has been maintained by continually testing 
and selecting the "mother" beets for the next crop of seed. How- 
ever, this necessity for continual selection does not exist in respect to 
all important field crops although they are subject to the general law 
of variation. That this must be so is clear when we realize that many 
natural species as well as cultivated varieties of plants are really mix- 
tures of sub-species, varieties, or races and that upon being isolated 
these distinct forms reproduce their own particular type. This is most 
easily demonstrated in plants normally self-fertilized^ yet in all naturally 

14 



VARIATION 15 

cross-fertilized plants and in higher animals this same endless diversity 
among individuals is even more marked. 

The Variation Concept. — As we have implied in the above remarks 
the term, variation, may be used in very different senses in referring to 
different phenomena. Thus variation within a species or variety means 
that the group in question is heterogeneous. Among individuals varia- 
tion may consist of differences between members of the same generation 
or between parents and offspring. Even when thus restricted, however, 
the term is apt to prove ambiguous. Hence it is necessary to give some 
thought to the sources, nature and causes of these individual differences 
in order that we may use clear cut expressions which shall always convey 
to one another a concept of the same particular sort of organic difference. 

Classification of Variations. — 1. Heritahility. — Character differences 
either represent something specific in the germ or they are merely the 
effect of external stimuli upon the individual soma. In the first case 
they are inherited, although they will not reappear necessarily in all 
later generations or in all the progeny. In the second case they will not 
be inherited. This is a fundamental distinction and may well serve as 
our primary basis of classification. According to heritahility variations 
are either germinal or somatic. Under germinal variations we recognize 
two sub-classes, combinations and mutations. Purely somatic variations 
will be referred to hereafter as modifications. 

Modifi.cations are non-heritable differences between the individuals 
of a race caused by the unequal influence of different environmental 
factors. Such variations frequently approximate continuity and, when 
studied statistically, display the normal variability curve, which w;ill be 
explained in the next chapter. 

Combinations are heritable differences between the individuals of a 
race or between the offspring of a pair of parents caused by segregation 
and recombination of hereditary units. They also frequently display 
the normal variability curve. 

Mutations are heritable differences between parents and offspring 
which do not depend upon segregation and recombination. 

These three categories, as Baur has shown, are not to be recognized 
and separated merely according to appearances. The cause of any 
individual differences can usually be established only by careful breeding 
experiments; but by this means the separation of the three categories 
is always possible as the boundaries between them are quite sharp. Modi- 
fications are somatic effects of environmental differences and should not 
be confused with germinal changes which, are sometimes induced by 
natural or artificial means and which result in the production of muta- 
tions. Within this first category must be included all place-effects in 
plants and somatic environmental effects in animals. Modifications 



16 GENETICS IN RELATION TO AGRICULTURE 

comprise a large portion of what are commonly spoken of as fluctuations 
due to environment, hut all cases of fluctuating variation are not modifica- 
tions inasmuch as variations due to combinations frequently display the 
normal variability curve also. Modifications .are not heritable. The 
second category, variation by combination of hereditary units is often 
confused with modification, as already stated, because of the fact that 
variations caused by segregation and recombination when studied statis- 
tically often display the normal variability curve. This is especially 
apt to be the case in quantitative characters (those of size or weight) 
and segregation and recombination may be the cause of gradations in 
color intensity. In autogamous (self-fertilized) organisms hybridization 
between races is sufficiently rare to be negligible n this connection, i.e., 
in such species the fluctuating variations are caused by the environment. 
But in allogamous organisms (those in which two individuals are neces- 
sary to accomplish sexual reproduction) fluctuating variations may be 
caused either by the environment, by segregation and recombination of 
factors, or by both causes acting together. We shall take up the third 
category, mutations, in a later chapter. For the present it is sufficient 
to remember that mutations are no doubt the least frequent of the three 
classes, that easily distinguishable mutations are comparatively rare, 
but that there may also occur true mutations of such moderate extent, 
as compared with the population, that their existence would only be 
detected by breeding tests, since their progeny would exhibit a different 
range of fluctuation from that of the population. 

2. Nature. We may next enquire into the nature of variation as 
it affects the organism. Upon this basis we may distinguish between 
four classes: morphological, physiological, psychological and ecological. 

Morphological variations are differences in size and form (Fig. 1). 
In general morphological variations have more significance for the biolo- 
gist than for the agriculturist. However in many products of the 
farm, size and conformation are of decided importance. Two sub-classes 
under morphological variations are meristic and homeotic variations. 
Meristic variations are differences in number of repeated parts such as 
the petals in a flower, the leaflets in a compound leaf or number of 
phalanges. Homeotic variations are differences caused by the replace- 
ment of one part by another, as the production of an antenna in place 
of an eye in an insect. 

Physiological variations are differences in quality and performance. 
Examples of qualitative variations are difference in degree of hardness 
of bone, flavor of meat, richness of milk, difference in normal color 
(Fig. 2), resistance to drouth, frost or alkali. Variations in performance 
constitute the most important group for the producer. Differences in 
performance are sometimes, though not necessarily, associated with 



VARIATION 



17 



certain details of structure. For example, note the prominent milk veins 
on the udder of Tilly Alcartra as shown in Fig, 231. 

Psychological variations are differences in mental traits. That mental 
and nervous conditions have very definite effects upon physical con- 




FiG. 1. — Morphological variation in number, form and size of leaflets in the blue elderberry, 

Sainbucus glauca. 

ditions is well known, but the problem of distinguishing ])etween pur- 
poseful action and automatic response, between manifestations of reason 
and manifestations of instinct, is set for the students of animal behavior. 
While variations in mental characteristics have an important place in 
eugenics and merit the attention of livestock breeders, yet the inheritance 



18 



GENETICS IN RELATION TO AGRICULTURE 



of pyschological characters must be more extensively investigated before 
the subject can be considered with profit in a fundamental study of 
genetics. 

Ecological variations are those differences between individuals that 
result from their fixed relation to the environment. These differences 
are especially noticeable in plants and are known as place-effects or 
place variations. This category includes some of the phenomena of 




Fig. 2. — Substantive variation due to chlorophyll reduction in certain areas of the leaves 

of Elasagnus pungens. 

variation in crop yield and hence is of immediate significance to agricul- 
ture. Fig. 3 illustrates place-effects in a common weed. 

3. According to differences between them there are two general classes 
of variations: first, the slight differences in every character which are 
always to beobserved even among individuals of identical heredity; second, 
unusual, striking differences commonly known as sports. The first class 
are called normal, indefinite fluctuating or continuous variations and the 
second, abnormal, definite and discontinuous variations. It should be 
noted, however, that all discontinuous variations are not necessarily 
definite or even distinguishable. Continuous variations when examined 
statistically are found to conform to the law of statistical regularity. 



VARIATION 



19 



That is, if measured and plotted the graph will approximate the normal 
curve of variability (Chapter III). Continuous variations are either 
heritable (combinations) or non-heritable (modifications) and, as was 
stated above, the only certain method of determining the class in which a 




Fig. 3.— Place-effects in common mustard (Brassica campestris) due to soil differences 

(herbarium specimens). 

given case may fall is the breeding test. Discontinuous variations are 
essentially discrete differences whether they be large or small. They 
are also either heritable or non-heritable and there is no correlation 
between size and heritability. Thus the extremely large and small 



20 GENETICS IN RELATION TO AGRICULTURE 

mustard plants shown in Fig. 3 considered by themselves are discontinu- 
ous variations, but they are almost certainly due entirely to environ- 
mental differences and seed from the small plant if grown under optimum 
conditions would produce plants of normal size. On the other hand, it 
is known that many minute differences in organisms are heritable. 

4. According to direction variations are classed as orthogenetic and 
fortuitous. Orthogenetic variations are those differences found in indi- 
viduals related by descent which form progressive series tending in a 
definite direction. Many remarkable illustrations are found among 
paleontological records of the evolution of animals. Occasional examples 
are found among short-lived or vegetatively propagated species. The 
remarkable series of variations of the Boston fern described in Chapter 
XVI is a good example. Fortuitous variations are chance differences 
occurring in all directions. 

5. According to cause variations are either ectogenetic, differences 
arising from conditions acting upon the organism from without; or 
autogenetic, differences resulting from strictly internal relations between 
germ and soma. 

Variation" and Development. — Somatogenesis, in sexually produced 
multicellular organisms, includes the entire history of cellular multipli- 
cation and specialization from the first cleavage of the fertilized (or 
parthenogenetic) egg to the completion of all adult features. From the 
standpoint of individual development it includes gametogenesis, for the 
production of sexual glands and of secondary sexual characters are merely 
phases of differentiation. Cell growth and cell function depend directly 
upon the activity of the living substance within the cell. The nature 
and degree of this activity depends upon two sets of determining causes 
acting simultaneously. First, there are the specific hereditary determiners 
or genetic factors, which react with the other elements of the protoplasm 
and, under favorable circumstances, condition normal development. 
Second, there are all the conditions external to the cell which stimulate or 
inhibit protoplasmic activity. These " developmental stimuli " are chem- 
ical and physical changes wrought by energy from without the organism or 
caused by its own physiological activities. Chemical stimuli are exerted 
mainly through the medium of the circulating liquid which surrounds 
each living cell. Normally this fluid contains the elements essential for 
maintenance of life as well as various waste products. It may also bear 
toxic substances that suppress or inhibit the cell functions and in higher 
animals it contains the secretions of the ductless, sexual and other glands 
that profoundly affect development. Physical stimuli are exerted 
chiefly from without and upon the organism as a whole. They include 
changes in temperature, light and density of medium, the effects of 
electric and radiant energy, force of gravity, etc. Obviously, so many 



VARIATION 



21 



interrelated causes acting simultaneously, ^ach being independently 
capable of inducing a change in the end product, may cause an infinite 
number of differences in substance and in degree of development. 

Variation and Environment. — External stimuli affect the develop- 
ment of characters in three ways: (1) they modify the development of 
inherited characters; (2) they actually condition the production of charac- 
ters whose hereditary determiners ai'e present in the germ-plasm; (3) 
they may cause germinal variations which result in the appearance of 
new heritable characters. The following arc illustrations of these effects 
with reference to particular environmental factors. 




Fig. 4. — Sedum spectahile. The three shoots (taken from a single plant) were planted in 
small pots on March 12, 1904, and placed in different greenhouses: /, in blue light; II, in 
mixed white light; III, in red light. Photographed on Sept. .30, 1914. {After Klebs.) 



1. Environment Modifies Development of Inherited Characters. — 

(a) Light and Function. — Klebs reports the results of growing the Showy 
Sedum (Sedum spectahile) in white, red and blue light. The diverse 
effects of the three kinds of light are clearly shown in Fig. 4. Although 
the visible differences between the three plants were very pronounced 
the experiment was carried much further. During 1905-06 observations 
were made on the numbers of stamens in the flowers of plants similarly 
propagated under white, red and blue light and under various conditions 
of temperature, moisture, and food. About 20,000 flowers were examined 



22 



GENETICS IN RELATION TO AGRICULTURE 



Substance 


White 


Red 


Blue 


Ash 


13.20 

n.04 

22.29 
0.16 
5.82 
5.33 


13.20 

15.40 

18.02 

0.33 

3.66 

6.15 


18.60 


Sugar 


2.40 


Calcium malate 

Free nitrogen ...... 

Starch 


18.10 
0.59 
1.20 


Crude protein 


7.64 



and six distinct types were found, according to the variation in number 
of stamens. These had the following average numbers of stamens: (1) 
9.68, (2) 8.45, (3) 6.54, (4) 5.05, (5) 9.47, (6) 7.33. Finally, Klebs 
subjected similar plants from white, red and blue light to chemical 
analysis in order to secure further evidence of the physiological effects 
of light of different wave lengths. Table I shows the composition 
of the leaves in three plants like those shown in Fig. 4. They were in 
their respective greenhouses from June 6 to September 7. The percent- 
ages shown are per 100 g. of 
Table I.-Chemical Composition of Three . substance. In compar- 
Plants op Sedum Spedabile Grown in . ,, , •. 

White, Red and Blue Light. ^^g these percentages it 

should be remembered that 
the plant in white light pro- 
duced 1324 flower buds and 
the plant in red light 405, 
while the plant in blue light 
produced none. This ex- 
plains the higher percentage 
of ash, nitrogen and protein 
in the last. On the other 
hand, the amounts of starch and sugar found in the plant from 
white light are decidedly larger than the one from blue light. In 
short, according to Klebs, in comparison with normal white light, 
the production of organic substances, such as starch and sugar, 
is diminished under the influence of blue light as microchemical 
and macrochemical tests distinctly show. In consequence of this di- 
minished assimilation of carbon dioxide the rosettes become purely 
vegetative. In red light the carbon assimilation is greater than in blue 
light but less than in white. These experiments prove that the transfor- 
mation of a plant "ripe to flower" into a vegetative one is possible on 
the one hand by an increase of temperature and of inorganic salts and 
on the other hand by a decrease of carbon assimilation. 

(6) Temperature and Pigmentation. — Many experiments in the rearing 
of moths and butterflies under controlled temperatures prove that degree 
of pigmentation is profoundly influenced by the temperature at which 
the pupae are kept. Some species exhibit seasonal dimorphism in the 
wild state. By taking pupse of the common European form of the 
swallowtail butterfly, Papilio machaon, and subjecting them to a tempera- 
ture of 37° to 38°C., Standfuss obtained the characteristic summer form 
which occurs in Palestine. Again it has been shown by temperature 
experiments that many variations found among insects in nature are 
merely aberrations due to temperature effects. Goldschmidt by arti- 
ficially controlled temperatures has produced a series of forms of the 



VARIATION 



23 



diurnal peacock butterfly, Vanessa io, which show the fading out of the 
"peacock eye" mark (see Fig. 5). 

(c) Food and Structure. — Woltereck was able to prove that the form 




Fig. 5. 



-The diurnal peacock-butterfly (Vanessa io), above, and below, forms produced 
by subjecting the pupse to unusual temperatures. (After Goldschmidt.) 



(hence the structure) of the fresh water crustacean, Hyalodaphnia, varies 
directly with the food supply. These minute animals produce many 
generations' during a season and the successive generations from the same 




28-VI 



30-VII 



15-IX 



Fig. 6. — Morphological cycle of head-height and shell-length in Hyalodaphnia. Roman 
numerals designate months. {After Woltereck, from Goldschmidt.) 

water exhibit a morphological cycle, the earher and later generations 
having shorter heads and the generations produced from midsummer to 
autumn having longer ones. Fig. 6 is a reproduction of Woltereck 's 
diagram of the morphological cycle in Hyalodaphnia showing variation 



24 



GENETICS IN RELATION TO AGRICULTURE 



in head and shell length as found on successive dates from June 3 to 
January 3. By raising these animals under constant temperature condi- 
tions and varying the strength of the nutrient solution, Woltereck proved 



35 



'40 



50 



55 



60 



70' 75 
m 9 



80 



85 



90 95 



Fig. 7. — Schematic curves of head-height in Hyalodaphnia as grown in media of three 
different food values. {After Woltereck from Goldschmidt.) 

that the relative size of body parts varied with the food. In Fig. 7 the 
percentages of head height to\shell length are plotted as abscissas and 
the numbers of individuals as ordinates. Animals from three strengths 





^^1 


^^^1 


^^^^bK$^ '^^^^I 


■■ 


BH 


^nk 


Ifl 




■ .' ^'i V 


f.^ 






^1 




^B - \ ^H 


ji V ^' if'^l 


ttPyi^^^B 


K.' 


-1 


fe *tjl 




> Er> * ''/*'^^B 


MSpS.'iSDj^^^H 




1 




■vfl 


i 


9 



Fig. 8. — a, Typical wild pigeon, Scardafella inca; b, the form dialeucos; c, hraziliensis; 
d, ridgwayi; e, S. inca after three moultings in a moist atmosphere. (After Beebe from 
Goldschmidt.) 

of nutrient media were measured, the curves of those from the weaker, 
the medium and the richer media being shown at Wi, m2 and ms 
respectively. 



V ART AT ION 25 

(d) Moisture and Plumage Color. — Beebe experiineiited witli the pigeon, 
Scardafella inca. This species, as found in North and Central America, 
is very constant in color of plumage, but in the moist tropics the following 
darker colored forms occur : in Honduras, dialeucos; in Venezuela, ridgwayi; 
in Brazil, braziliensis; and these differ in the amount of pigment in the 
feathers. By subjecting birds of the northern type to an especially 
moist atmosphere, Beebe caused them to be so influenced that with each 
new moulting, whether natural or artificially induced, they always de- 
veloped darker feathers. Thus a wild bird having ])igm('nt in 25.9 per 
cent, of its area, would have after the second moulting under experimental 
conditions, 38 per cent, and after the third, 41.6 per cent. Thus during 
the experiment the typical form assumed the appearance of the three other 
forms and finally developed plumage markings which have never been 
seen in nature. Fig. 8 shows the type form, inca, the three geographical 
variants, and the darkest artificially produced form. 




Fig. 9. — Plants of Scilla, stalled alike hul (lie pot uii tlie riiiht was kept in a ilaik room. 

{From Ganono-) 

2. Environment Conditions Development of Inherited Characters. — 

(a) Light and Metabolism. — In a general sense light conditions life in all 
normally green plants. It certainly conditions normal development in 
such plants. Potatoes sprouted in a dark room develop no chlorophyll 
in the stems and the rudimentary leaves are abortive. In many bulbous 
plants, however, the influence of moisture and heat are sufficient to 
induce leaf growth and even development of the inflorescence, but it 
is all done at the expense of the food stored up in the bulb as is shown 
in Fig. 9. 



26 GENETICS IN RELATION TO AGRICULTURE 

(6) Temperature and Flower CoZor.—B aur reports an experiment with 
a red variety of the Chinese primrose, Primula sinensis rubra. If 
plants of this variety are raised by the usual method until about one 
week before time to bloom and then some of the plants are put in a warm 
room under partial shade (temperature from 30° to 35°C.) and the re- 
mainder in a cool house (temperature from 15° to 20° C), when they bloom 
those in the warm temperature have pure white flowers while those in the 
cool temperature have the normal red color of the variety. Moreover, 
if plants are brought from the warm into the cool temperature the flowers 
which develop later on will be normal red in color. Thus it cannot be 
said that this primula inherits either red or white flowers. What it 
really inherits is ability to react in certain ways under the influence of 
temperature. 

(c) Food and Fertility. — It is well known that the kind of food supplied 
to the larvae of bees determines whether the females shall be fertile 
(queens) or infertile (workers), (Fig. 10). The striking differences in 




Fig. 10. — The three forms of bees: a, drone; b, queen; c, worker. The two latter develop as 
the result of difference in the food supplied to the larvae. {After Harrison.) 

structure and instincts of the two classes of females are all conditioned 
by the food provided for the larvae. Each larva inherited the capacity 
to react in either way according to the stimulus received. 

(d) Moisture and Structure. — Morgan reports a variety of the pomace 
fly, Drosophila ampelophila, with abnormal abdomen (Fig. 60); "the 
normal black bands of the abdomen are broken and irregular or even 
entirely absent. In flies reared on moist food the abnormality is extreme ; 
but even in the same culture the flies that continue to hatch become less 
and less abnormal as the culture becomes more dry and the food scarce, until 
finally the flies that emerge later cannot be told from normal flies. If 
the culture is kept well fed (and moist) the change does not occur but 
if the flies are reared on dry food they are normal from the beginning." 

3. Environment May Cause New Heritable Characters. — As yet 
there is a dearth of evidence which can be accepted as scientific proof 
that external stimuli actually cause germinal variations. At the same 
time there is an abundance of data which falls into the class of circum- 
stantial evidence in favor of such a doctrine. Moreover, there are a few 



VARIATION 27 

cases in which new heritable characters have been artificially produced 
by carefully controlled external stimuli. Hence some germinal variations 
are apparently caused by known environmental conditions and we are 
justified in recognizing this third category of developmental differences 
due to environmental effects. 

Considerable evidence of permanent changes in both morphological 
and physiological characters has been secured from experiments with the 
culture of bacteria and yeast, in unusual culture media, in the presence 
of toxic solutions, or under extreme temperature conditions. The sig- 
nificant results of four investigators who worked independently, Hansen, 
Barber, Wolf and Jordan, have been reviewed and discussed in regard to 
their bearing on genetic theory by Cole and Wright. The four investi- 
gators mentioned above used refined methods and three of them began 
by isolating a single organism from whose progeny they obtained dis- 





Fio. 11. — 0, Portion of leaf of parental Scrophularia showing branching lateral vein; 
D, branching vein replaced by two laterals in leaf of a seedling grown from seed produced by 
an injected ovary. Also note difference in size and margin of leaves. (After Mac Dougal.) 

tinct strains or biotypes which remained constant for hundreds of test- 
tube "generations." It must be admitted that in most of these cases 
no specific influence can be named as the direct cause of the inherited 
variation. But there is no longer any doubt that permanent, discon- 
tinuous variations do occur spontaneously in these lowest organisms, and 
it is highly probable that certain incidental, external forces play an im- 
portant part in inducing such variations. 

Direct experimental attack upon the germ cells themselves has 
been made with plants by a number of investigators, notably by Mac- 
Dougal, who injected very dilute solutions of potassium iodide, zinc 
sulphate, sugar, etc., directly into the ovaries of various plants imme- 
diately before fertilization. Consequently somatic changes have been 
produced which were inherited throughout several generations. By 
means of check experiments and observations it was found that these 
germinal variations were not caused by the wounding of the ovary and it 
is thought that they must have been induced in some way by the presence 
of the foreign chemical solution in the ovary. Fig. 11 shows a mor- 
phological change which appeared in a seedling of an unnamed species 



28 GENETICS IN RELATION TO AGRICULTURE 

of Scrophularia as a result of ovarial injection. Having tested this 
species sufficiently to determine that it was a simple one, MacDougal 
treated several ovaries with potassium iodide, one part in 40,000 and se- 
cured seed. No other species of Scrophularia grew near the cultures. 
From this seed only three plants were raised. "One formed a shoot 
fairly equivalent to the normal, finally producing flowers in which the 
anthocyans were of a noticeably deep hue. The two remaining plant- 
lets were characterized by a succulent aspect of the leaves and by a 
lighter and yellow color of the leaves and stems. The flowers on one of 
the derivatives, as they may be called, were so completely lacking in 
color as to be a cream-white, this derivative being designated as albida, 
while the other showed some marginal color and a rusty tinge and was 

designated as rufida Seeds of the original two derivatives 

were sowed in the greenhouse. But one plant of albida, the most extreme 
departure, survived, while four of rufida were secured." MacDougal 
compared these second generation seedlings with seedlings from the 
original stock of the species, noting differences in size and margin of 
leaves, length of petioles and number of marginal glands. He found 
that the differences shown by the first generation appeared again in the 
second generation. Striking as these results appear it must be admitted 
that it would be difficult, on accoufit of the small numbers of individuals 
differing from the parent type, to prove satisfactorily to the biome- 
trician that they were not mutations which would have occurred regard- 
less of the ovarial treatment. 

What appear to be germinal variations in the tomato have been induced 
by intensive feeding. T. H. White tested the effect of dried blood, dis- 
solved phosphate rock, sulphate of potash and iron filings all in excessive 
amounts, and (with the exception of the iron) in various combinations, on 
the Red Cherry tomato. The lack of data on control cultures of seedlings 
from the same parent as the experimental cul tures makes it impossible to 
compare the actual amount of permanent variation produced. T. H. 
White states that measurements ''show that the plants of the sixth gen- 
eration grown under the influence of the dried blood are one-third larger in 
height, length of leaf and size of fruit, than those of the second"; (see 
Fig. 12). The author concludes that " there can be no doubt . . . that, 
in the case of Red Cherry treated with dried blood, there is permanent 
variation to the third generation." If these results are corroborated by 
more carefully planned and rigidly controlled experiments they will add 
the weight of scientific proof of a principle in plant breeding long since 
recognized on empirical grounds, to wit, that the introduction of wild 
plants into intensive cultivation induces variation. Furthermore, it 
suggests a possible means for rapid permanent improvement of wild 
forms with which hybridization may be impracticable. 



VARIATION 



29 



In experiments on lower animals, e.g., the protozoa, the same difficulty 
is met with as has been encountered in bacteria and yeasts, in that it is 
manifestly impossible to distinguish between somatic and germinal 
variations. Moreover, in most of these experiments, as with most of 
those on higher animals, the necessary conditions for rigid scientific 
analysis have been lacking. Either the same strain as was subjected to 
artificial conditions was not grown for comparison under natural condi- 
tions or else the conditions themselves were not sufficiently well con- 




FiG. 12. — Leaf and cluster of fruit of Red Cherry tomato of the second generation 
(right); same of the sixth generation (left) of continuous treatment with excessive amount of 
dried blood. {Photo by T. H. White.) 



trolled to permit of certain analysis. It is interesting to note that the 
pomace fly, Drosojjhila ampelophila, which has produced more mutations 
so far as we know than any other organism, was subjected to the effects 
of ether on a grand scale and under controlled conditions by Morgan, 
but that not a single mutation was observed to result from this treat- 
ment. However, mutations have subsequently appeared again and 
again in cultures of " wild " flies not only of this species but also of other 
species of Drosophila. Thus it appears that germinal variations fre- 
quently occur independently of external stimuli. It also seems that a 
tendency to produce mutations may be inherited. 



30 



GENETICS IN RELATION TO AGRICULTURE 



With animals the best known experiments on the artificial production 
of germinal variations are those of Tower who worked with the Colorado 
potato beetle, Leptinotarsa decemlineata, and related species. Like other 
arthropods these beetles are more directly under the influence of tempera- 
ture changes at least than are warm-blooded animals. Tower first de- 
termined the period in ontogeny when external stimuH will affect the 
germ cells. He found that in Leptinotarsa the germ cells do not 
become susceptible to external stimuH until after the time in ontogeny 
when the color pattern of the individuals subjected to the stimuli can be 
influenced. He found that eggs were most susceptible just before and 
during maturation and this observation is in agreement with those of 
Fischer, Standfuss, Weismann and others who have conducted similar 




Fig. 13.- 



-A, Leptinotarsa decemlineata and three mutants; B, tortuosa; C, pallida; D, 
defectopunctata . (After Tower.) 



investigations. Tower concluded that certain individuals from the germ 
cells of a stimulated parent "show intense heritable variations, whereas 
those not acted upon do not show these changes. " Most of the inherited 
variations involve changes in the pigmentation of the body parts. In 
certain cases there was an actual change in the color pattern (see Fig. 
13). It is to these results that Tower attaches the greatest significance 
inasmuch as most similar experiments have not succeeded in causing 
pattern changes. In spite of the elaborateness of Tower's methods con- 
siderable skepticism exists regarding the validity of his conclusions, and 
this has not been lessened by the non-appearance of confirmatory data. 
In a recent paper he reports the production of very striking germinal 
modifications in L. decemlineata as a result of subjecting a morphologically 
homogeneous race to an extreme change in environment. However, it 
is still a question whether the material used may not be heterogeneous 
as regards the germinal factors that condition certain physiological 
characters. 

Stockard's investigations on the effect of alcohol on the progeny of 
guinea pigs have shown that the germ cells as well as the somatic tissues 



VARIATION 31 

of the alcoholized animals are injured. This case is considered further 
in Chapter XXX. 

On the whole it must be admitted that the experimental induction 
of heritable variations is still largely an unworked field. The complex 
conditions to be considered and consequent obstacles to be overcome 
are appreciated by no one more fully than by those who have attempted 
such investigations. For, as Tower has said: "It is evident that the 
problem of germinal change is one of difficulty, and involves more of 
indirect than of direct methods of investigation. There is little reason to 
expect that present biochemical methods can give a solution, but they 
may give valuable suggestions for further indirect investigation. It 
seems not improbable, however, that this problem like so many others 
in biology, must await the solution of the larger question of what life is 
before it will be possible to express in exact terms the nature of germinal 
changes. Our present status, with several methods of production and 
much knowledge of the behavior of induced germinal changes available, 
is a basis from which great advances in knowledge and in operation may 
reasonably be expected." 



CHAPTER III 
THE STATISTICAL STUDY OF VARIATION 

In the present chapter we shall consider the applicatior^ of purely 
statistical methods in the analysis of biological phenomena especially the 
phenomena of variation. The treatment given here does not pretend to 
be exhaustive or rigorous, but it presents the commonly used method 
of recognized biometricians, from several of whom valuable suggestions 
have been received. We shall have occasion to refer to the utilization 
of statistics in the study of heredity by the "biometrical school," but 
the application of statistical methods in the analysis of specific genetic 
problems will be deferred until later chapters. 

Causes of Fluctuations. — Continuous variations, or the slight differ- 
ences normally found in organisms, are generally referred to as fluctuating 
variations or fluctuations. It is frequently assumed that "fluctuating 
variability" is due entirely to differences in environment. But, as was 
stated in the preceding chapter, either the modifications in development 
due to environment, or individual differences which are caused by seg- 
regation and recombination of genetic factors, may display the normal 
curve of variation when examined statistically. Hence fluctuations 
may be due to either of two causes and before conclusions may be drawn 
from the study of frequency distributions and statistical constants, the 
causes of the variations studied must be clearly differentiated. The only 
way to accomplish this is to make one set of conditions or the other as 
uniform as possible. If the object be to examine modifications, only 
pedigree material should be used and, on the other hand, if variations due 
to recombinations are to be considered, the environmental conditions 
must be as uniform as possible or else due account must be taken of exist- 
ing irregularities. Certain technical requisites to the biometrical method 
will be mentioned later. This difference in the nature of fluctuating 
variations according to their cause is of such fundamental importance 
that it should be clearly understood at the outset. 

Law of Statistical Regularity. — This fundamental principle, which 
is also known as the law of probability or law of chance, may be most 
simply introduced by means of an illustration. Suppose two persons, 
blindfolded, were each to pick about 500 beans from a bag containing a 
million beans of any standard variety. The average weights of the beans 
picked out by the two persons would be almost identical even though the 

32 



THE STATISTICAL STUDY OF VARIATION 



33 



individual beans varied considerably in size. Furthermore if one were 
to obtain the average weight of the whole million, it would not differ, 
essentially, from the average weights of the smaller groups. The prin- 
ciple involved here may be stated in various ways. Wold expresses it 



M=I5.4 




// U 13 /+ 13 /(. n 19 19 iO i/ JA 



Fig. 14. — Frequency distribution of 500 Broad Beans arranged in classes according to 

width. 



as follows: "If a number of different events are equally possible as regards 
constant conditions {that is, if there is no persistent reason why one should 
occur rather than another), and all are repeatedly given opportunity to 
occur, they will in the long run occur with equal average frequency .^^ While 
this is a satisfactory general statement of the law of probability, the same 



34 GENETICS IN RELATION TO AGRICULTURE 

principle has been expressed by King in terms, which fit well the imaginary 
case under discussion, as follows: ''A moderate^ large number of items 
chosen at rundown from among a very large group are almost sure, on 
the average, to have the characteristics of the large group." It must not 
be inferred that any partial group of individuals no matter how large, 
will give exactly the same results as would be obtained by the use of the 
entire mass. But the averages will be close and the probability of in- 
accuracy due to accidental errdr diminishes as the numbers increase 
because individual errors tend in the long run to counteract each other. 

Law of Deviations from the Average. — If, now, one lot of 500 beans 
be measured to the nearest millimeter and then arranged in columns from 
left to right according to width beginning with the narrowest beans, the 
result will be very similar to Fig. 14. It will be noticed first that the 
middle classes contain the most beans while the classes on the extreme 
left and right are very small. The black vertical line M indicates the 
average width or mean of all the beans and the column with the most 
beans in it represents the most frequent width of beans and is called 
the mode. The columns nearest the average value on either side contain 
the most beans and the further the column is from the average the fewer 
the beans in it. Thus we see that the majority of the beans show 
only slight deviations from the average while a few exhibit wide deviations 
therefrom. Statistical study has proved that it is a general rule with 
fluctuations that individuals showing extreme deviations in either 
direction for a given character are comparatively rare, while individuals 
exhibiting smaller deviations, and hence occupying a position inter- 
mediate between the two extremes are especially frequent. In other 
words, continuous variations usually appear in frequencies such that, 
if we represent these frequencies graphically, we obtain a polygon which 
resembles more or less the normal variability curve. Such a polygon 
is produced by connecting the ends of the columns in Fig. 14. 

The Normal Curve and its Significance. — ^The normal variability 
curve is a theoretical curve which pictures the result of expanding the 
binomial (a + 6)" when a = b = 1 and n is assumed to be indefinitely 
great. By the binomial theorem 

(a + 6)1 =1 + 1 

(a + 6)2 = 1 + 2 + 1 

(a + 6)3 = 1 + 3 -F 3 + 1 

(a + 6)4 =1+4-1-6 + 4 + 1 

(a + 6)5 =1+5 + 10 + 10 + 5 + 1 

(a + 6)6 =1+6 + 15 + 20 + 15 + 6 + 1 

{a + hy =1+7 + 21+35 + 35 + 21+7 + 1 

(a + 6)8 =1 + 8 + 28 + 56 + 70 + 56 + 28 + 8 + 1 

(a + 6)9 = 1+9 + 36 + 84 + 126 + 126 + 84 + 36 + 9 + 1 

(a + 6)10 = 1 + 10 + 45 + 120 + 210 + 252 + 210 + 120 + 45 + 10 + 1. 



THE STATISTICAL STUDY OF VARIATION 



35 



From Fig. 15 it is evident that as n becomes larger the straight lines of 
the polygon more closely approximate the normal curve. 

The normal curve is perfectly symmetrical because it represents the 
distribution of an indefinitely large number of items and it assumes all 
causes to be of equal strength or value. It is assumed that certain 
biological frequency polygons should simulate this curve for these reasons. 
It is probable that the environment of any organism is made up of a 
large number of factors each of which may vary around a mean independ- 




\ 



Fig. 15.- 



-Polygons representing expansion of the binomials (a + b)^ and (a -j- b)^° as 
compared with the normal curve. 



ently of the others. Now if a frequency polygon is to be made regarding 
a character of a population composed of individuals alike in zygotic 
constitution, such as a field of potatoes of the same variety, the differences 
found in the development of any character are due wholly to these en- 
vironmental factors. Hence it is likely that the mean of the distribution 
is made up of observations on individuals upon which an equal number 
of favorable and unfavorable forces have acted and the deviates are those 
upon which a greater or less number of favorable or unfavorable forces 
have acted. But in sexually reproduced allogamous species the in- 
dividuals are not alike in zygotic constitution. Moreover, the causes 
affecting a given character may have an unequal mass effect according to 
ecological conditions. Either of these factors may cause a high degree of 
asymmetry in a polygon of variation. Graphs in which the mode is 
rather far removed from the mean are called skew polygons or curves. 



36 



GENETICS IN RELATION TO AGRICULTURE 



The significance of the normal curve as an index of variation is based 
on the conception that the area within the curve represents an indefinite 
number of individuals and that the constants of the curve indicate the 
distribution of these individuals with respect to a given character. If 
in any curve (Fig. 16) the perpendicular erected at M divides the area of 
the curve into two equal parts, this line is the median and the point M 
represents the average or mean of all the values from which the curve is 
constructed. The perfect symmetry of the normal curve causes the 
median to coincide with the mean and the mode; but in actual cases 





Fig. 16. — A normal curve divided 
into quartiles by the perpendiculars 
erected at M,Qi,Qz- 



Fig. 17. — A normal curve of exactly the 
same area as the curve in Fig. 16, but with flat- 
ter slope and correspondingly greater breadth. 
The distribution pictured by this curve pre- 
sents a greater range of variation than in 
Fig. 16 as is indicated also by the value of Q. 



'these three values will not coincide because the curve will not be sym- 
metrical. If a perpendicular be erected in either half of the curve at 
such a distance from M that it divides the area enclosed by the median, 
the base and half of the curve into two equal parts, the distance of such 
a perpendicular, Qi or Q^ from M is the quartile, q. Then in the normal 
curve q = MQx = MQ^. Now the slope of the curve is an index of the 
amount of variability. The steeper the slope supposing the area (the 
number of individuals) to remain the same, the nearer to the median 
will be the position of the quartile and hence the position of the quartile 
is also an index of variability (Fig. 17). Since curves constructed from 
actual distributions are never symmetrical, in practice the index taken is 

— n However, the measure of variation in common use is the standard 

deviation, <r, which in the normal curve represents a distance from the median 

equal to ^^^' 

Requirements of Biometrical Study. — The data for statistical analysis 
are obtained by counting, by measurement, or by arbitrary graduation 
of continuous differences like degree of pigmentation. In order that such 



THE STATISTICAL STUDY OF VARIATION 37 

data may be compared with other similar data some sort of precise de- 
scription must be prepared. Graphical representation is good as far as it 
goes ; a frequency polygon conveys to the eye more knowledge than one 
would have without it. But in order to secure the best description of 
organisms with reference to specified characters, some mathematical 
expression for the degree of variation must be deduced from the data. 
This process involves two essential steps: (1) To obtain a measure of 
type for the group under observation; (2) to derive an expression for the 
amount of variation from the type. There are three measures of type, 
the median, the mode and the mean, and we have seen that in the theo- 
retical normal curve they always coincide. In actual cases they may be 
widely separated. There are three commonly used measures of variation 
from type, viz., the range, or the distance from one extreme to the other, 
the quartile, and the standard deviation. These expressions and others 
derived from them are known as the constants of the normal curve. In 
practical work the meoM, or arithmetical average, is commonly used as 
the measure of type and the standard deviation as the absolute measure 
of variation. A relative index of degree of variation is derived by divid- 
ing the standard deviation by the mean; this is called the coefficient of 
variation. These three constants are the indispensable mathematical 
tools of the biometrician. Some knowledge of their calculation and 
significance is necessary for an intelligent appreciation of considerable 
important biological and agricultural literature. Before proceeding to 
discuss these constants it will be necessary to present a few technical 
terms and methods. 

Some Biometrical Terms.^ — An Individual may be either an entire 
organism or only a single part as the leaves of a tree or seeds of a plant. 
Individuals are also called variables. 

A Sample is any group of individuals which are measured or com- 
pared with a standard. Samples may be divided into sub-samples for 
definite reasons; for example, corn from different parts of a field. 

The Population is the general mass or entire group from which sam- 
ples are taken. 

A Variate is a single magnitude-determination of a character. 

A Class includes variates of the same or nearly the same magnitude. 
The class range gives the limits between which the variates of any class 
fall. 

Requisites to Reliability. — 1. Biological Soundness. — Three great 
sources of untrustworthiness in biological work are: 

(a) Differences due to age; different ages must not be lumped to- 
gether without taking account of it. 

(b) Heterogeneity due to conditions of environment; for example, 
corn from a field in which the soil is definitely heterogeneous. 



38 GENETICS IN RELATION TO AGRICULTURE 

(c) Mixing of distinct varieties, which must never be permitted if 
known in advance. 

2. Definition of Population. — The population must be so defined that 
conclusions reached will not be wrongly apphed to other populations. 

3. Typical Sample. — The sample must be really typical of the species, 
variety, breed, strain or race. Otherwise the results are notapphcableto 
large populations. Also the sample must be large enough so that con- 
clusions may be drawn fairly. 

4. Sufficient Accuracy. — Measurements must be made with a suf- 
ficient degree of accuracy. It might be thought that a coarse or slightly 
variable scale of measurement would satisfy since the measurements 
are to be grouped, but the relative size of the groups is a most critical 
matter so that the size of scale and degree of accuracy are very important. 
Yet perfect accuracy is hardly obtainable. Relative not absolute ac- 
curacy is the desideratum. As stated by King: For every statistical 
problem there should he determined in advance a definite standard of 
accuracy for each item and every endeavor should be made to bring each 
recorded instance up to this standard. 

Grouping Variates into Classes. — When the individuals have all been 
measured the collection of variates must be grouped. The following 
rules should be observed: 

1. Classes should be of equivalent ranges. One must not neglect 
the extremely large and small variates. Employ a uniform scale through- 
out all classes. 

2. Arrange the classes so there will be no possibility of mistake by the 
reader. Calculations may be based on the centers of the class intervals 
or on the upper limits of the intervals for certain purposes. 

The Frequency Table.— A list of the classes formed by the grouped 
variates together with the number of individuals in each class is called a 
frequency table. For example. Love and Leighty give the data on total 
yield of plant in grams of Sixty Day oats for the year 1910 at Ithaca, 
N. Y. • These are presented in the form of a frequency table in Table II. 



Table II. — Frequency Table Showing Variations in Yield of Sixty Day 
Oats. {After Love and Leighty) 

f) (Class value = V) (Frequency = /) 

ants ' Grams of oats Number of plants 

5-6 = 5.5 42 

6-7 = 6.5 7 

7-8 = 7.5 2 

8-9 = 8.5 1 



(Class value = V 
Grams of oats 


(Frequei 
Number 


0-1 = 0.5 


3 


1-2 = 1.5 


50 


2-3 = 2.5 


106 


3-4 = 3.5 


109 


4-5 =4.5 


80 



Total number of individuals 400 = n 



THE STATISTICAL STUDY OF VARIATION 



39 



Frequency Graphs. — To graphically represent the data in the above 
frequency table, indicate a base line on a sheet of coordinate paper, mark 
off equidistant points for class intervals and midway between the limits 
of each class indicate the class center. In this case the class intervals 
are 0-1, 1-2, 2-3, etc., and the class centers are 0.5, 1.5, 2.5, etc. Counting 
each space above the base line as one or more individuals (according 



109 



106 



M= 3. 458 + .045 
a =1.323 +.032 
C =38.259 ±1.037 




Fig. 18. — Frequency polygon showing variation in total yield per plant in grams of 
Sixty Day oats at Ithaca, N. Y., 1910. (Data from Love and Leighty.) 

to the modal number and size of sheet), either construct rectangles of 
proper altitude to represent the frequency of each class or merely indicate 
the points of intersection of the frequencis plotted as abscissas and 
the class centers as ordinates. The latter method is usually employed 
since it is more rapid and the polygon more truly represents the 
distribution of classes in a sample showing continuous variation in the 
character in question. This method is illustrated in Fig. 18. The area 
within the polygon represents the actual data for which purpose a curve 
should never be employed. 



40 



GENETICS IN RELATION TO AGRICULTURE 



The Mean, Calculation and Significance. — To compute the mean of a 
series of single variates summate the variates and divide by the number of 
variates. Thus if x = any variate and n = the number of variates, then 



the mean, M, = 



X(x) 



where S indicates summation. 



For a series of groups of variates (classes), first multiply each class 
value (7) by the number of variates in the class or frequency (/) then 
summate and divide by n. Thus 



Table III. — To Compute the Mean 
Total YieI;D of Plant in Grams 



V 


/ 


f.v 


0.5 


3 


1.5 


1.5 


50 


75.0 


2.5 


106 


265.0 


3.5 


109 


381.5 


4.5 


80 


360.0 


5.5 


42 


231.0 


6.5 


7 


45.5 


7.5 


2 


15.0 


8.5 


1 


8.5 




n = 400 


S = 1383.0 



In the calculation of this and other constants it is important that the 

work be indicated in a systematic 
manner. The form as indicated 
in Table III is usually preferred. 
The data are the same as in the 
frequency table (Table II). 

A valuable short method of 
computing the mean consists in the 
use of an assumed mean which 
removes the necessity of multiply- 
ing the class values by their fre- 
quencies and hence greatly reduces 
the actual labor in dealing with 
large numbers. For the same data 
the short method is shown in Table 
IV. The rule is as follows: To 
compute the mean of a series of 
classes of variates, write the fre- 
quency of each class in a column on the right of the class values, then 
the deviation of each class from an assumed mean, and lastly the product 
of each deviation by its corresponding frequency. Summate the devia- 
tion-by-frequency products, divide by n, and add algebraically the correc- 
tion factor thus obtained to the assumed mean (in this case, 3.5). 

— 17 
Correction factor = w = ^7^-= —0.0425 

M = 3.5 + (- 0.0425) = 3.458. 
Thus for the computation of the mean by the short method we have 
the formula 

n 
The mean is the best measure of type in organisms because it takes into 
account all the individuals measured. For this reason the sum of the 



M = ^1 = 3.458. 
400 



THE STATISTICAL STUDY OF VARIATION 



41 



variations from the true mean of all the items in the table equals zero. Unlike 
the mode it is affected by every item in the group so that its location 
can never be due to a single class; moreover it gives weight to extreme 

deviations. The measure 

Table IV. — To Compute the Mean Total Yield 
OF Plant IN Grams. Let G = assumed mean = 3.5 



V 


/ 


V-G 


/(V-G) 


0.5 


3 


-3 


- 9 


1.5 


50 


-2 


-100 


2.5 


106 


-1 


-106 -215 


3.5 


109 








4.5 


80 


1 


80 


5.5 


42 


2 


84 


6.5 


7 


3 


21 


7.5 


2 


4 


8 


8.5 


1 . 


5 


5 198 




n =400 




-17 



of type used and its value 
should always be indicated 
on a graph. For a precise 
description of the variation 
within a group it is neces- 
sary to have something 
more than a measure of the 
type. Knowing the arith- 
metical average is not 
sufficient to permit com- 
parison of the variation in 
different populations. 
There is needed some mea- 
sure of variability. 

The Standard Deviation, 
Calculation and Significance. — Examination of the original records 
of weighings of the total yield of the 400 oat plants would reveal 
a certain amount of variation in the yield of each plant from the 
mean yield, 3.458 g. The plants were grouped into classes in com- 
puting the mean yield and they can be treated similarly in calculating 
the average amount of variation from the mean jaeld for the whole 
sample. It may be noted that the simplest measure of the absolute 
variation within the sample is the average deviation, which is simply cal- 
culated by summating the products of the deviation of each class from 
the true mean multiplied by its frequency and dividing this sum by n. 
The standard deviation is universally preferred as an absolute measure of 
variability. The standard deviation differs from the average deviation 
in one important feature, viz., that in calculating the standard deviation 
each individual variation from the mean is squared. This gives addi- 
tional weight to the extreme variations which is especially desirable in 
biometrical work. 

In calculating the standard deviation (Table V) the regular procedure 
is as follows : Write the minus and plus deviation (d) of each class from the 
mean, square each deviation (d^), multiply each d^ by the frequency (/), 
summate the products, divide by n and extract the square root. This 
is expressed by the formula 



-^i 



2(/.d2) 



42 



GENETICS IN RELATION TO AGRICULTURE 



Table V. — To Compute the Standard Deviation in Mean Total Yield op Plant 
IN Grams (Complete Process Including Calculation of the Mean) 



V 


/ 


f-v 


d 


d^ 


f.d^ 


0.5 


3 


1.5 


-2.958 


8.750 


26.250 


1.5 


50 


75.0 


-1.958 


3.854 


191.700 


2.5 


106 


265.0 


-0.958 


0.918 


97.308 


3.5 


109 


381.5 


0.042 


0.002 


0.218 


4.5 


80 


360.0 


1.042 


1.086 


86.880 


5.5 


42 


231.0 


2.042 


4.170 


175.140 


6.5 


7 


45.5 


3.042 


9.254 


64.778 


7.5 


2 


15.0 


4.042 


16.338 


32.676 


8.5 


1 


8.5 


5.042 


25.442 


25.442 




n = 400 


S(/.F) = ,1383 
M = 3.458 






2(/.d2) = 700.392 



V 



700.392 



400 



1.316. 



Table VI. — To Compute the Standard Deviation by the Short Method 
Let assumed mean = G = 3.5; V — G = d' 



V 


/ 


d' 


f.d' 


f.d'^ 


fid' + ly 


0.5 


3 


-3 


- 9 




27 


12 


1.5 


50 


-2 


-100 




200 


50 


2.5 


106 


-1 


-106 


-215 


106 





3.5 


109 













109 


4.5 


80 


1 


80 




80 


320 


5.5 


42 


2 


84 




168 


378 


6.5 


7 


3 


21 




63 


112 


7.5 


2 


4 


8 




32 


50 


8.5 


1 


5 


5 


198 


25 


36 




n = 400 




-17 
400 


0.0425 


"» - 1.7526 
400 


1067 



M = G -\- w w^ = 0.0018 

= 3.5 + (-0.0425) 1.7525 - 0.0018 = 1.7507 
= 3.458 <r = vTTSO? = 1.323 

Check: 2(/) + 2S(/.d') + S(/.d'^) = 1067 = S[/K + 1)^] 

The short method for computing the standard deviation is based upon 
the same principle as the short method for the mean. The rule, therefore, 
is as follows: Select some number approximating the mean (G); write 
the minus and plus deviation therefrom (rf') ; multiply each deviation 



THE STATISTICAL STUDY OF VARIATION 43 

by the corresponding frequency (f.d') ; divide the difference between the 
minus and plus products by n to obtain correction factor (w); then 
multiply each f.d' by d' to get f.d'^; summate the last products and 
divide by 11; from the quotient subtract w^ and then extract the square 
root. The illustration, Table VI, is based upon the same data as the 
preceding. 

It will be noted that this value of the standard deviation is slightly 
larger than the value as computed by the regular method. The short 
method is the more accurate because of the elimination of many decimal 
places. In additon to the complete short method there is shown in the 
last column on the right a very useful method of checking the computa- 
tion. Each f{d' + 1)^ is calculated algebraically. Thus in the first 
case / = 3 and d' = —3; substituting we have 3( — 3 + 1)^ = 12. In 
the same way S(/) + 2^{f.d') + ^{f-d'^) is computed algebraically. 
Substituting we have 400 + (-34) + 701 = 1067. 

The standard deviation, being a measure of absolute variation, is 
exceedingly useful in comparing the variability of one variety with 
another with respect to the same character, or of the same variety in 
different years with respect to a given character, or of one character with 
another in the same or different species. For example. Love and Leighty 
in their memoir on "Variation and Correlation of Oats" give the means , 
and standard deviations for total yield of plant in grams (as well as for 
eight other characters) for the same pure strain of Sixty Day oats for 
three years as follows: 

1909 - M = 4.032, a = 2.249 

1910 - M = 3.458, (X = 1.323 
1912 - M = 7.962, a = 3.353. 

The differences between these values are due mainly to differences in 
climatic conditions during the three years, the year 1910 having been 
especially dry and hot. Similar differences appear in the means and 
standard deviations for height of plant, number of culms and number of 
grains produced. This particular observation leads to no new con- 
clusion as it is well known that climatic conditions profoundly influence 
crop yield, but it illustrates the significance of the standard deviation 
as a measure of variation. Furthermore it is of interest to note that 
drouth not only reduces plant growth and yield in this variety but the 
amount of variation as well. 

In 1910 the amount of absolute variation was only one-third that of 
1912. However, the amount of relative variation was not so much 
affected by drouth as might at first appear. When comparing standard 
deviations of different varieties or of the same variety under diverse 
conditions, it should be remembered that the means of the groups under 



44 GENETICS IN RELATION TO AGRICULTURE 

consideration may be widely different in value. It may even happen 
that the characters to be compared were measured in different units, as 
inches and grams. Hence it is desirable to have an expression of vari- 
ability in relation to the mean. Such an expression is the coefficient of 
variability which is the ratio of the mean to the standard deviation ex- 
pressed in per cent. The formula for the coefficient of variability is 

lOOo- 



C = 



M 



In the case of total yield of plant in grams for Sixty Day oats in 1910 
substituting the values which have been calculated we have 

100 X 1.323 
^ ~ 3.458" ~ ^^•^^^' 

The coefficients for the other two years are: 1909, 55.779 and 1912, 
42.113, Thus the amount of relative variation in yield was much 
greater in 1909 than in 1912 and although the standard deviation for 
1910 is only a third as large as that for 1912, yet the amount of relative 
variation is almost as great. A measure of absolute variation is very 
useful but a relative measure is essential, especially when comparing 
different kinds of material such as total yield in grams and number of 
culms or milk production and butter fat production. 

The Theory of Error. — It has been said that the frequency curves of 
many biological measurements follow the curve made by plotting the points 
given by the expanded binomial (a + 6)" where a = 6 = 1. The reasons 
why this should be true are not difficult to see. They depend upon the 
laws of probability or chance that have been generalized into the theory 
of error. The chance of an event happening in an infinite number of 
trials is expressed by a fraction of which the numerator is the number 
of ways it may occur and the denominator is the total number of ways 
it may occur or fail to occur, if each is equally likely. Thus in tossing a 
coin a great number of times, the chances that it falls heads is one-half. 
Further, the probability that all of a set of independent events will 
occur on a single occasion in which all of them are in question is the product 
of the probabilities of each event. Hence, the probability that two coins 
tossed together will fall heads is 3^^ X .^^ = 3^. 

Now suppose four coins are tossed at random ; what is the probabihty 
that any particular number m of them will be heads and the rest tails? 
The number m may be 0, 1, 2, 3, and 4, and the probabiHties are as 
follows : 

head and 4 tails = l(Ji)* 

1 head and 3 tails = 4(>^)4 

2 heads and 2 tails = 6(K)* 

3 heads and 1 tail = 4(3.^)* 

4 heads and tail = liH)*. 



THE STATISTICAL STUDY OF VARIATION 



45 



The coefficients that appear are what they are because precisely those 
combinations are possible. There is but one combination in which there 
are no heads, there are four combinations consisting of 1 head and 3 
tails, there are six combinations possible of 2 heads and 2 tails, there 
are four combinations of 3 heads and 1 tail, and again but 1 with no tails. 
But this is simply the expansion of the binomial (1 + !)■*. The prob- 
ability that when n coins are tossed exactly m of them will be heads 
and the rest tails, therefore, is given by the m + 1st term of the binomial 
expansion (1 + 1)". When n is small a symmetrical frequency 
polygon is obtained somewhat similar to that given by plotting the yields 
of individual oat plants. When n is very large more points are obtained 




Fig. 19. — A normal curve or curve of error showing the relationship between the quar- 
tile, i.e., the probable error of a single variate, and the standard deviation. Q = .6745<r. 
In this curve the mode, median and mean are identical. The quartile equals the probable 
error of a single variate because by definition one-half of the variates lie within its limits; 
therefore the chances are even that any variate lies within or without it. The proportions 
of variates within certain areas of the curve are as follows: 

within M ± Q, 50 % of the variates, within M ± a; 68.3 % of the variates, 
within M ± 2Q, 82.3 % of the variates, within M ± 2<r, 95.5 % of the variates, 
within M ± 3Q, 95.7 % of the variates, within M ± 3<r, 99.7 % of the variates. 

and the polygon becomes a regular curve, the normal probability curve 
or curve of error. It is called the "curve of error" because if a refined 
set of direct measurements are made and plotted as abscissas, the corre- 
sponding ordinates represent the frequencies or probabilities that each 
will occur. The mean is the most probable value and is assumed to be 
the true value and the deviations from the mean are errors. Positive 
errors lie to the right and negative errors lie to the left of the mean. 
Positive and negative errors are equally likely to occur if they are gov- 
erned by chance only and as the errors increase in magnitude the 
frequency with which they occur becomes less and less. 

Let us assume that we have a perfectly normal frequency curve such 
as that represented in Fig. 19, and we shall be able to demonstrate the 
meaning of some of the constants that we have learned to calculate for it. 
This curve represents observations on a large number of individuals and 



46 GENETICS IN RELATION TO AGRICULTURE 

its area represents the general distribution of these individuals . The mean 
represents the average of the distribution. The standard deviation 
(plus and minus) represents the ordinates of those points on the curve 
where the slope changes from convex to concave; it therefore measures the 
slope of the curve and is a good measure of its variability. Measuring 
from M to 0- on each side of the curve, we find that the space enclosed 
includes 68.3 per cent, of the total number of individuals; within the limits 
+ 2cr lie 95.5 per cent, of all individuals and within ± Za He 99.7 per 
cent. Thus we see that although theoretically the curve never meets 
the ground Hne but extends out to infinity, practically all individuals are 
found within the limits ± So-. 

Similarly we find that the quartile measures the number of individuals 
within the Hmits of the curve that it marks off as follows: 

M ± Q includes 50.0 per cent, of the individuals 
M ± 2Q includes 82.3 per cent, of the individuals 
M ± 3Q includes 95.7 per cent, of the individuals 
M ± 4:Q includes 99.3 per cent, of the individuals 
M ± 5Q includes 99.9 per cent, of the individuals. 

In a normal curve, therefore, the standard deviation and the quartile 
have a constant relationship such that Q = 0.6745(r. 

From these relationships an idea of the meaning of the term "prob- 
able error" which is always calculated for any series of observations may 
be obtained. The probable error tells us what confidence we may place 
in our work, if the errors are due to chance only and not to avoidable 
mistakes of method. The probable error is not the "most probable error." • 
The most probable error is and hence is identical with the mean. 
Probable error is an arbitrary term used to denote the amount that must 
be added to or subtracted from the observed value to obtain two limiting 
figures of which it may be said that there is an even chance that the true 
value lies within or without these limits. 

The probable error, E, of a single variate is the quartile,^ since the 
chances are even that any variate lies within or without the value M -\-Q; 
and since 82.3 per cent, of the variates lie within the value M + 2Q, the 
chances are 4.6 to 1 that the true value of any series of a calculated con- 
stant is within these limits. Thus the chances that the true value lies 
within any multiple of E are 

+ E the chances are even 
± 2E the chances are 4.6 to 1 

^ The Germans use a as the measure of error. It is known as the error of mean 
square and is proportionately larger than the probable error as is shown by the fact 
that 

within M ± o- lie 68.3 per cent, of the variates 
within M + 2<T lie 95.5 per cent, of the variates 
within M + 3o- lie 99.7 per cent, of the variates. 



THE STATISTICAL STUDY OF VARIATION 47 

± SE the chances arc 21 to 1 
± 4:E the chances are 142 to 1 
± 5E the chances" are 1310 to 1 
± 6^ the chances are 19,200 to 1. 

Since biometricians use the standard deviation as the measure of 
variabihty, the relation between it and the quartile is utiUzed in deter- 
mining all probable errors, even though there is some real error in such 
a proceeding due to the distribution scarcely ever being exactly normal. 
The probable error of the mean is found by multiplying the standard 
deviation by 0.6745 and dividing by the square root of the number of 

±0.6745cr 
variates, thus E,„ = — y~ — ' Hence the reliability of the determi- 
nation of the mean increases not in proportion to the number of 
variates but in propoi'tion to the increase of their square roots. 

The probable errors of the standard deviation and the coefficient of 
variability are as follows, but it is not necessary here to go into the proof 
of the determinations. 



E<x = 



±0.67450- 



\/2n 

)^745Cr ,^/Jl_\' 

V2n [^^^[lOOj 



+ 0.6745Cr / C 



_ ±0. 6745(7 

\/2n 

approximately if C is not greater than 10 per cent, because, if the group 
of variates approximates a normal frequency distribution, the value of 
C will be less than 10 per cent, and the value of the quantity within the 
brackets will approximate unity and so can be neglected. 

The significance of probable error is most apparent when comparing 
statistical results; for example, the standard deviations for average total 
yield of plant in two or more varieties. Concerning the significance of 
probable errors Rietz and Smith make the following statement: 

In the comparison of two statistical results, the difference between 
the two results compared to its probable error is of great value. In 
general, we may take the probable error in a difference to be the square root 
of the sum of the squares of the probable errors of the two results. If the 
difference does not exceed two or three times the probable error thus 
obtained, the difference may reasonably be attributed to random sam- 
pling. If the difference between the two results is as much as five to ten 
times the probable error, the probability of such differences in random 
sampling is so small that we are justified in saying that the difference is 
significant. In fact a difference of ten times its probable error is certainly 
significant in so far as there is certainty in human affairs. 



48 



GENETICS IN RELATION TO AGRICULTURE 



Multimodal Curves. — Thus far we have considered only homogeneous 
populations, which, when examined statistically, exhibit a certain degree 
of approximation to the normal curve of variation. Populations fre- 
quently occur, however, both in nature and among domesticated animals 
and plants, which are found to be heterogeneous for certain characters 
at least when subjected to statistical analysis. Graphically shown the 
data for such a character produces a polygon with more than one mode. 
In general such data indicate either the permanent influence of different 
causes affecting only certain individuals or of the same cause acting 
differently upon a portion of the population. Conditions of bimodal 
curves are more or less familiar to all. Sexual dimorphism and certain 




3 4 5 6 7 8 9 

Fig. 20. — Bimodal polygon plotted from data on the earwig. Mean types ( X %,) 
indicated above corresponding modes. Numbers below the base line indicate length of 
pincers in mm. {From Bateson and Johannsen.) 

differences in development which are contingent upon sex, such as height 
of comb in fowls, obviously would result in a "notched" graph if the 
characters were measured and the data plotted. The classic example of 
dimorphism producing a bimodal curve is found in the length of the 
pincers of the common earwig (Forficula auricularia) as reported by 
Bateson. Fig. 20 illustrates the two mean types, each sketch being 
placed directly above its corresponding modal class in the graph. Other 
conditions commonly causing mixed populations such as would result in 
bi- or multimodal curves are the following : 

1. Coexistence of groups of different ages; common in birds at certain 
times of the year. 

2. Overlapping of geographical races of the same species — birds, 
mammals. 

3. Coexistence of different races of the same species, for example, 
many grasses in the wild state and various cultivated grains contain 



THE STATISTICAL STUDY OF VARIATION 49 

several or many different races. Variation in the number and propor- 
tion of these races in the population would produce wide differences in 
statistical data. 

4. Germinal diversity among the individuals of a population due to 
hybrid ancestry. 

Analj'-sis of the causes contributing to bi- and multimodal curves is 
possible by means of experimental breeding. By testing individuals 
typical of the various groups indicated by the statistical examination 
and examining their progeny statistically, the elements composing the 
original population can be differentiated. It should be noted that the 
close proximity of two different races sometimes causes contamination 
of material and consequent skewness of the variation polygon but not 
necessarilj^ a bimodal curve. 

Correlation. — All of the biometrical principles considered in the pre- 
ceding pages pertain to the analysis of variation in a single character. 
One of the most striking facts of somatogenesis, however, is the physio- 
logical interdependence of characters in multicellular organisms. From 
the earliest stages of embryogeny it is possible to trace associations in 
the development of various characters. This physiological correlation 
of characters is one of the most important considerations in the modern 
study of heredity and it is given due attention in Chapter VI. As regards 
the statistical study of variation the question to be considered is whether 
the continuous variations in adult somatic characters are in any cases 
mutually related or interdependent. It is obvious that, if such a condi- 
tion be found to exist, it will have an important bearing upon plant and 
animal breeding inasmuch as selection with reference to a single character 
would in all likelihood have a definite effect upon certain other characters. 
The most satisfactory method of investigating this matter is to consider 
the variation in two characters at a time. 

The Correlation Table. — ^In preparing a correlation table the observed 
data are transferred directly from the original record by the simple method 
of tallying. In order to prepare a correlation table either the indi- 
viduals to be examined must be labelled with permanent numbers or 
else the observation on the two characters must be made for each indi- 
vidual before passing on to the next. In either case the datum on each 
character is recorded under the individual number. Next a table is ruled 
off with a number of horizontal rows corresponding to the total number of 
class values for one of the characters and a number of columns equal to 
the total number of class values for the other character. It is under- 
stood that a frequency table for each of the two characters has been 
previously prepared so that the range of class values is known. In 
Fig. 21 the material examined consists of the same 400 plants of Sixty 
Day oats that we have studied with reference to total yield of plant. 



50 



GENETICS IN RELATION TO AGRICULTURE 



The character of yield is now to be considered in relation to the number 
of culms per plant. Hence there will be nine rows and seven columns 
in the correlation table. The class values are indicated in consecutive 
order beginning usually at the upper left-hand corner. In the present 
instance, oat plant No. 1 yielded 0.5 g. and had 2 culms, hence this plant 



No. of 
indi- 
vidual 


Yield 
in gm. 

Vx 


Vy 


1 


0.5 


2 


2 


2.5 


3 


3 


6.5 


5 


4 


4.5 


4 


5 


1.5 


4 


6 


2.5 


7 


7 


7.5 


5 


8 


0.5 


2 


9 


8.5 


6 


10 


5.5 


4 


11 


1.5 


3 


12 


1.5 


4 


.3 


2.5 


3 





2 


3 


4 


5 


6 


7 


0.5 

1.5 

2.5 

3.5 

4.5 

i 5.5 

6.5 

: 7.5 

i 8.5 


11 














1 


11 










11 








1 


















1 














1 












1 












1 














1 





Fig. 21. — To illustrate transference of data from original record to correlation table. 
Vx indicates class values for total yield of plant, Vy, number of culms per plant. 

is tallied in the upper left-hand square of the table; plant No. 2 yielded 
2.5 g. and had 3 culms, it is tallied in row 3 column 2, and so on throughout 
the list of 400 plants. Then the tallies in each square are counted, re- 
corded and transferred to a new table drawn on a smaller scale for future 
use, the original table being filed as a permanent record. In this way 
the tables shown in Figs. 22 and 23 were prepared. 

Interpretation of the Correlation Table. — A correlation table is a 
record of the frequency distributions for two different characters so 
arranged as to show the tendency, if any exists, for one character to 



THE STATISTICAL STUDY OF VARIATION 



51 





2 


3 


4 


5 


6 


7 


0-1 


3 












1-2 


28 


19 


3 








2-3 


18 


66 


20 


1 




1 


3-4 


1 


42 


58 


7 


1 




4-5 




7 


59 


11 


3 




5-6 






26 


14 


2 




6-7 








4 


3 




7-8 






1 


1 






8-9 










1 





50 



134 



167 



38 



10 



3 

50 

106 

109 

80 

42 

7 

2 

1 

400 



Fig. 22. — Correlation table for 400 plants of Sixty Day oats. Total yield of plant 
in grams, subject. Number of culms per plant, relative. 1910. Coefficient of correla- 
tion = 0.712 ± 0.017. (From Love avd Leighiy, 1914.) 





2 


3 


4 


5 


6 7 


45-50 




1 


1 






50-55 


2 


3 


3 




1 


55-60 


4 


9 


4 


2 


2 


60-65 


6 


10 


13 


4 


1 


65-70 


8 


40 


42 


6 


1 


70-75 


15 


41 


53 


12 


2 . 


75-80 


10 


22 


43 


11 


3 


80-85 


4 


8 


8 


3 


1 


85-90 












90-95 


1 











50 



134 



167 



38 



10 



2 

9 

21 

34 

97 

123 

89 

24 



1 



1 400 



Fig. 23. — Correlation table for 400 plants of Sixty Day oats. Average height of plant 
in centimeters, subject. Number of culms per plant, relative. 1910. Coefficient of 
correlation = 0.042 + 0.034. (From Love avd Leighty, 1914.) 



Vy - Mu = -d. 



Vy — My - dy 



y 



Vs- M^ = -d. 



V^- M, = d. 





{ — dx)i— dy) =dxdy 
(2) 


(dx)(— dy) = — dxdy 
(3) 


(1) 

(-dx)(d„) = -djy 


(4) 

idx){dy) = dxdy 



Mx 



Fig. 24. — Interpretation of the correlation table. 



> X 



M. 



52 



GENETICS IN RELATION TO AGRICULTURE 



change as the other character changes. The general features of such a 
table are shown in Fig. 24. The intersection of the two means Mx and 
My, divides the table into quadrants, which are numbered 1, 2, 3, and 
4. The signs of the deviations from the mean of x and y are opposite 
in the 1st and 3d, while they are the same in the 2d and 4th quadrants. 
Now the deviation from M of every individual in the table is Vx — Mx 
in terms of x and Vy — My in terms of y. As these deviations are to 
be considered relatively, their products are taken. The products of 
unlike signs are negative, 1st and 3d, and of like signs, positive, 2d and 
4th. After arranging the x and y individuals in arrays, if the larger 
number fall in the 1st and 3d quadrants, we learn that there is negative 
correlation or a tendency for one character to diminish as the other 



/ 


\ 


1 

\ 


1 

1 
1 
1 

1 



Fig. 25. 



-Interpretation of the correlation table. Shape of 
and amount of correlation. 



'swarm" indicates nature 



increases. If the majority fall in the 2d and 4th quadrants, we conclude 
that there is positive correlation or a tendency for one character to in- 
crease as the other increases. If the individuals are uniformly distributed 
in the four quadrants we find no evidence of interdependence i.e., zero 
correlation. These typical distributions are illustrated by the three 
diagrams in Fig. 25. Comparing the two correlation tables (Figs. 22 
and 23) with these diagrams it is evident that the correlation between 
yield of plant and number of culms is definitely positive, while the nature 
of correlation (whether positive or negative) between average height of 
plant and number of culms cannot be inferred from mere observation 
of the table but that it is very low indeed is clear from the widely scattered 
distribution. 

The Coefficient of Correlation. — The interpretation of a correlation 
table is based upon the fact that the table shows deviations with respect 
to two characters for each individual or class of individuals. We must 
remember that the x and y deviations of each class from the mean are 
multiplied in order to understand how the distribution in the table can 
indicate plus, minus, or zero correlation between the characters. The 
product of the two deviations for any individual or class is its product- 



THE STATISTICAL STUDY OF VARIATION 



53 



moment, and the summation of all the product-moments divided by 7i 
is the average product-moment. This measure of absolute correlation is 
expressed by the formula 



Av. prod. -mom. = 



Mdxdy) 



No. of culms per plant > x 

G. = i 

2 3 4 5 6 7 /„ 



f-'l'u 



f.d'-y :i(d'xrf'v) 



2- 



3- 



8-9 

/x 



f.d'. 



SA'"- 



28 



18 



19 



66 20 



42 



50 
-2 



-100 



200 



134 



-134 



134 



58 



59 



26 



14 



38 



40 



50 



106 



109 



80 



42 



-2 



■100 



•106 



80 



84 



21 



27 



200 



106 



80 



164 



63 



32 



25 



400 Wy = 



-17 



400 
= -.0425 
wK= .0018 



-^173 
400 



= -.4325=i/'x 
1871 



421 

= 1.0525 

400 j^871 

.8654 
<rx=.9303 



697 
400 
1.7425 
.0018 
1.7407 

ay = \. 323 



18 



150 



98 



10 



30 



30 



10 



-3(-6) 

-2[-56 + (-19)] 
-l[-36 + (-C6) + l+3] 

l(-7+ll+0) 
2(14 + 1) 
3(4 + 6) 
4(1) 
5(2) 



356 



= .890 
400 

WxWy= .0184 
.8716 



.8716 



Er= + 



(1.323) (.9303) 
6745(1 -r2) 



= . 708 



■y/n 
.6745(1 -.5013) 

20 

rx»=.708±.017. 



= +■ 



.0168 



Fig. 26. — Calculation of coefficient of correlation (r) for total yield of plant in grams and 
number of culms per plant for Sixty Day oats grown at Ithaca, N. Y., in 1910. 



But we cannot compare such a number as derived for instance from 
size of potatoes and starch content with size of beets and sugar content 
without reducing them to a relative basis. Pearson suggested the product 
of the two standard deviations as the best index of variability by. which to 
divide the average product-moment so as to reduce it to a relative basis. 



54 GENETICS IN RELATION TO AGRICULTURE 

His formula is the one now generally used. If the coefficient of corre- 
lation equal r, 
Then 






We know the work of computing the standard deviation is lessened 
by using the short method. Hence this method should be employed in 
computing the correlation coefficient. On the basis of assumed means 
from which the deviations are d'x and d'y we have 



\ n I \(TxcrJ 



from which we read the following rule: 

To compute the coefficient of correlation, multiply the x and y 
deviations from G for each class; summate the products and divide by 
n; from the quotient subtract the product of the two correction factors; 
divide this difference by the product of the two standard deviations. 

The application of this formula is based upon the correlation table 
and is illustrated in the case of total yield of plant in grams and 
number of culms per plant for Sixty Day oats (Fig. 26). 

Interpretation of the Coefficient of Correlation. — King gives the 
following rules for the interpretation of the coefficient of correlation 
according to its relation to the probable error: 

1. If r is less than the probable error, there is no evidence whatever 
of correlation. 

2. If r is more than six times the size of the probable error, the 
existence of correlation is a practical certainty. 

3. In cases where the probable error is relatively small: 

(a) If r is less than 0.3 the correlation cannot be considered at all 
marked. 

(6) If r is above 0.5 there is decided correlation. 

Applying these rules to the case of variation in yield as related 
to number of culms we see that r is over 40 times the probable error and 
under rule 3, the probable error being relatively small, since r = 0.7 + , 
there is very decided correlation. Referring now to relation of number 
of culms per plant to average height of plant (Fig. 23) we find that 
r = 0.042 + 0.034 from which it is clear that there is little if any indi- 
cation of correlation. 

Biometricians consider the correlation coefficient the most powerful 
tool the agricultural investigator can have since it is a most excellent 
measure and is applicable to an immense range of variables. Remember- 
ing that this constant is an index of the mutual relation that exists 
between the variations of any two characters, we realize that, if it is 



THE STATISTICAL STUDY OF VARIATION 55 

high, it indicates they are in some way closely related, and, if it amounts 
to unity it shows that one is the cause of the other or else both are the 
result of the same causes. The importance of biological soundness as 
a requisite to reliability in the correlation coefficient must not be over- 
looked, e.g., see Harris on physical conformation of cows and milk 
yield. Pearl reminds us that statistical knowledge of correlation is 
precise only in the same limited sense that similar knowledge of type 
and deviation from type is precise, viz., as applied to the particular 
group or groups in the particular instance in time. However, this ability 
to describe groups in terms of the groups' own attributes is extremely 
useful in the practical conduct of scientific experiments. Love and 
Leighty point out that correlations may be classified as fluctuating and 
stable, "these divisions being based on the behavior of the relationship 
of the characters concerned when variation occurs in environmental con- 
ditions, such as exist in different years, or in different locations. As the 
names indicate, the correlations of the first class may be made to vary 
considerably by changes in conditions, while those of the second class 
remain of about the same value or are stable in character." The prac- 
tical value of knowledge of correlation is great, especially when one char- 
acter is easily seen or readily measured and the other is not. Although 
it is difficult for the mind to grasp the relation which exists between two 
groups of data on several hundred or thousand individuals, yet when the 
relation between those data is expressed in a single number as a corre- 
lation coefficient the difficulty disappears. 

Regression. — The correlation between parents and offspring wKen 
used as a measure of inheritance — Galton thought his measure of somatic 
resemblance was a measure of inheritance — ^is usually known as regression. 
If in an allogamous species parents and offspring be compared with respect 
to the same character, it is found that the means of the offspring are 
nearer the mean of the general population of parents than they are to the 
mid-value of their own parents. In other words, extreme parents do not 
produce progeny as extreme as themselves. Galton believed this re- 
gression toward the mean of the general population to be due to "pull" 
of a mediocre back ancestry. He expressed a mathematical law, good 
under certain conditions, that is directly opposed to biological facts. 
It expresses the truth, that, if from a general population of mixed heritage 
in which there is continual crossing, extremes are selected as parents, 
there will be regression toward the mean of the general population; and 
continued selection will be necessary therefore to improve the race. 
But this regression is not due to the pull of a back ancestry; it is due to 
the fact that individuals whose somatic appearance places them in diverse 
classes in the frequency distribution are themselves gametically different 
and will breed differently. Circumstances may come about by which the 



56 GENETICS IN RELATION TO AGRICULTURE 

breeding efficiency is such that the regression will be negative — that 
is, away from the mean of the general population — as has been proved 
by Shull, by Emerson and by East in experiments with maize; Further- 
more, Johannsen, Jennings and others have shown that when the indi- 
viduals of a population are alike gametically and their differences are 
due to external conditions only, these differences are not inherited at all 
and regression is perfect. This means that if a number of beans are alike 
gametically, selection of extreme sizes will not shift the mean in either 
direction. 

More recently biometricians have applied the mathematical principle 
involved in Galtonian regression in order to express in absolute terms the 
relative interdependence of characters expressed by correlation coefficients. 
Used in this sense regression is commonly represented by a straight line 
which approximates the largest possible number of the subject means in 
a correlation table. The "regression straight line" is extensively used 
by some authors as a method of representing the relation between the 
absolute values of characters. For excellent illustrations consult Han-is 
on body pigmentation and egg production in the domestic fowl. 

Employment and Value of the Statistical Method. — It may be as- 
sumed that biometrical methods are not worth very much if the great 
biological generalities of the biometricians are misleading. Such an 
assumption would also be misleading. Statistical methods are a great 
aid to biologists, but they are only an aid. Trouble has arisen only 
when biological conclusions have been drawn by mathematicians who 
ignored certain biological premises. One caD only take out of his 
mathematical mill just what he puts in, but he can take it out in a more 
comprehendible form. If he has made an accurate biological analysis 
mathematics are a help; if he has made no biological analysis mathe- 
matics are a hindrance. Johannsen sums up the whole situation in the 
sentence: "We must treat genetic facts with mathematics not as mathe- 
matics." If the beginner is careful of his biological premises, if he is 
certain that the material with which he deals is representative — that he 
has a random sample — if he makes no mathematical deduction unjusti- 
fied by common sense analysis, he will find that the use of mathematics 
will remove many a rough place from his road. Biometry will always be 
an indispensable instrument for the scientific breeder as well as the 
geneticist. The agronomist and pomologist also have need to resort 
to statistical methods in order to reach a satisfactory solution of many 
problems involving variation such as variety testing, seed germination 
tests, investigation of the value of bud selection, etc. Intelligent em- 
ployment of the statistical method insures conservative and reliable con- 
clusions regarding many questions which would otherwise remain in the 
debatable class. 



CHAPTER IV 
THE PHYSICAL BASIS OF MENDELISM 

Recent investigations in heredity have focused attention upon the 
chromosome mechanism as the physical basis for the segregation and re- 
combination of the units of Mendehan inheritance. The importance 
of cytological phenomena to students of genetics is admirably summed up 
by E. B.Wilson in the brief statement that "heredity is a consequence of 
the genetic continuity of cells by division, and the germ cells form the ve- 
hicle of transmission from one generation to another." It is appropriate, 
therefore, to introduce the subject of Mendelism with a formal and 
brief treatment of the chromosome mechanism and its mode of operation, 
on the one hand, in the building up of the body from the single cell with 
which the individual begins its existence, and, on the other hand, in the 
production of germ cells when the individual reaches the reproductive 
period of its life cycle. It is the purpose of this chapter merely to deal 
with the fundamental facts of cytology which are necessary to an under- 
standing of the connection between cell behavior and Mendelian phe- 
nomena. Details unessential to such an understanding, however well 
established cytologically, will not be dealt with in this treatment to the 
end that the cardinal points may be presented as simply and as clearly 
as possible. 

The Chromosomes. — With few exceptions the number of chromosomes 
in the cells of any individual is constant and characteristic of the species 
to which the individual belongs. Thus it is characteristic of Drosophila 
ampelophila that the cells contain eight chromosomes. In maize the 
cells contain twenty chromosomes, in wheat sixteen, and in man forty- 
eight, and so on through the entire plant and animal kingdoms. 

Not only is the number of chromosomes in a particular species con- 
stant, but the chromosomes themselves possess a definite individuality. 
Man and tobacco have cells with the same number of chromosomes. It 
is needless to point out that these chromosomes, however, are quali- 
tatively very different. Similarly within the species the chromosomes are 
not all alike; on the contrary, especially in certain forms, they exhibit 
very marked differences in size and shape. This is peculiarly well illus- 
trated in Drosophila as shown in Fig. 27. Here it is possible to recog- 
nize in the female two large pairs of curved chromosomes very similar 
in size and shape. There is also a very small pair of chromosomes, and 

57 



58 GENETICS IN RELATION TO AGRICULTURE 

finally there is a pair of straight ones about two-thirds as long as the 
large curved chromosomes. In the male the same relations hold except 
that instead of the pair of straight chromosomes there is a pair consisting 
of one straight and one somewhat larger hooked chromosome. The 
significance of this difference in chromosome content in the sexes will 
be pointed out in a consideration of the inheritance of sex. The pair 
of straight chromosomes we call the sex or X-chromosomes, the unequal 
mate of the X-chromosome in the male of this species is called the Y- 
chromosome. The other chromosomes are called autosomes when it is 
desired to distinguish them as a class from the sex chromosomes. 
Drosophila is not unique in possessing chromosomes of such characteristic 

FEMALE HtLC 

Fig. 27. — Diagram showing the characteristic pairing, size relations, and shapes of 
the chromosomes of Drosophila ampelophila. In the male an X- and a F-chromosome 
correspond to the X pair of the female. On the basis oi X = 100 the length of each long 
autosome is 159, of each small autosome 12, of the whole Y 112, of the long arm of the Y 71, 
and of the short arm of the Y 41. {After Bridges.) ] 

shapes and sizes; but more and more as cytology advances it is 
becoming possible to distinguish individual chromosomes, and to 
recognize them at every cell division. 

Moreover, the characteristic paired relations which exist among the 
chromosomes of Drosophila are of general significance. When mature 
germ cells are formed in an individual, reduction divisions occur by means 
of which the chromosome number is reduced in the germ cells to one-half 
that characteristic of the body cells. Thus the germ cells of Drosophila 
contain four chromosomes as the result of a reduction which takes place 
in such a manner that each germ cell contains one member of each pair 
of chromosomes. As a consequence, the germ cell of Drosophila contains 
two large curved autosomes, representing the two pairs of these chromo- 
somes, one small autosome, and one X- or one F-chromosome. The 
same thing is true for other species of plants and animals — in the reduc- 
tion divisions the chromosomes are distributed in such a manner that 
each germ cell receives one member of each pair of chromosomes. It 
follows from this that in general a definite number of pairs of chromo- 
somes is characteristic of the body cells of individuals of a given species. 



THE PHYSICAL BASIS OF MEN DELI SM 59 

and, taking the chromosomes by pairs, one member of each pair is de- 
rived from one parent and the other from the other parent. 

From the standpoint of interpretation the chromosomes are aggre- 
gates of chromatic material which in itself is definitely and highly or- 
ganized. Our conceptions of this feature of cell organization are based 
on appearances of the cytological preparations from certain of the more 
favorable plants and animals and further interpreted by investigations 
on heredity. Accordingly the entire chromatin content of the nucleus 
is regarded as made up of a definite number of individual chromatin 
elements called chromomeres. The number of chromomeres in a cell of any 
species must run into the thousands. A certain definite group of these 
elements make up each chromosome, and at every cell division this chro- 
mosome is reformed from the same group of chromomeres. The individu- 
ality of the chromosome, therefore, depends on the individuality of 
the chromatin elements of which it is made up. Not only is each chro- 
mosome made up of a definite group of chromomeres, but the chromosome 
is definitely organized with respect to the position or locus occupied by 
each chromomere. At certain stages in the history of chromosomes, 
they are simply lines of chromomeres, very much like single strings of 
beads with each bead corresponding to a chromomere. Now it appears 
probable that all the chromomeres in a chromosome are different, as 
though our string of beads had no duplicates throughout its length. 
Moreover, each chromomere has a definite place or locus in the par- 
ticular chromosome in which it belongs and it is always found at that 
particular locus. The chromomeres of this discussion are identified with 
the factors of Mendelian heredity, and how closely this conception of 
the nature of chromatin and its complex organization corresponds to 
the modern view of Mendelian phenomena will be pointed out as each 
new phase of Mendelism is taken up. 

Somatic Cell Division. — 'The phenomena of cell division (called mi- 
tosis) are represented in outline in Fig. 28, for a species having four 
chromosomes in its body cell. Bearing in mind the description which 
has just been given of the organization of the chromatin material we may 
follow the steps involved in mitosis as they are outlined in this figure. 
In the "resting" cell at A the chromatin is scattered throughout the nu- 
cleus in clumps or knots loosely strung together to form an irregular 
network. As the cell prepares for division the chromatin elements 
appear in more definite form until at B the chromomeres have arranged 
themselves in a single row in a long, continuous spireme-thread. This 
spireme-thread may be considered to be made up of the four chromosomes 
united end to end with the chromomeres arranged in a hnear series. As 
mitosis progresses to the next stage represented at C, each chromomere 
of the spireme-thread divides into two so that a double spireme-thread 



60 GENETICS IN RELATION TO AGRICULTURE 

results from the longitudinal splitting of the original thread. Both parts 
of the thread are quantitatively and qualitatively equal, for, by the 
splitting of all the chromomeres both of the threads come to possess 
all of the individual elements of the original spireme thread. Following 
the splitting of the chromomeres and the formation of a double spireme, 
the spireme-thread contracts and segments transversely forming four 
double chromosomes, the number characteristic of the cells of this 
individual. This is the stage shown at C where also is shown the origin 
of the spindle, a part of the mechanism in mitosis. The chromosomes 
now still further contract until they assume their characteristic shapes 
and sizes. They next appear in an equatorial position on the spindle as 
shown at Z>, where the two pairs of double chromosomes, one larger 
and one smaller, . are diagrammed and the nucleolus, the large black 
body of the previous steps, is shown cast out and degenerating. The 
daughter chromosomes of each pair now separate from each other 
until at E they have moved nearly to the opposite poles of the spindle 
and are beginning to fray, out and seemingly to lose their identity. At 
this stage actual division of the cell body has begun. Finally at F, the 
chromosomes have completely lost all appearance of their identity, the 
chromatin material is distributed throughout the nucleus as in the origi- 
nal cell shown at A, and the nucleolus has been reformed in each nucleus. 
Division of the cell-body has resulted in two daughter cells each of which, 
so far as chromomeres are concerned, contains exactly the same chromatin 
elements as the original cell. 

There are many variations in this process particularly in the order of 
occurrence of the steps, but these variations in nowise modify the essen- 
tial fact of mitosis which is that the chromatin material of the cell is 
converted into a thread which splits throughout its entire length into two 
halves so that the daughter nuclei receive exactly equivalent portions 
of chromatin material. This precise division of the chromatin is brought 
about by a division of each chromomere so that not only do the daughter 
nuclei receive equivalent portions of chromatin but these portions are 
also equivalent qualitatively to the entire chromatin content of the 
mother cell. By this method then each of the cells of the body finally 
comes to possess not only the whole number of chromosomes contrib- 
uted by the two parents, but also the entire set of chromatin elements 
which it received from them. The extreme care with which the cell 
mechanism partitions the chromatin material in each successive cell 
division is in itself eloquent testimony of the fundamental importance 
of this material. 

The Production of Germ Cells. — In the production of germ cells a 
different set of phenomena occur which result in a reduction of this num- 
ber of chromosomes to one-half that characteristic of the somatic cells. 



THE PHYSICAL BASIS OF MEN DELI SM 



61 



Preceding the actual reduction division the chromatin material passes 
through a complex series of steps which may be included under the term 
synapsis. (This term is sometimes applied in a specific sense to the 
pairing of homologous chromosomes and sometimes to the contraction 
of the chromatin threads in the conjugation stage.) The essential steps 
in the prereduction process are shown in outline in Fig. 29. At A is 
diagrammed a "resting" nucleus at the completion of the multiplication 
divisions in the germ plasm. As a result of the exact type of mitosis 
which has been outlined above it contains the full number of chromosomes 
characteristic of the species. The chromatin of the nucleus next becomes 






Fig. 28. — Diagram of mitosis in a species having four chromosomes in its cells. A, The 
"resting" cell. B, Formation of the spireme-thread. C, Longitudinal division of the 
spireme-thread and transverse segmentation into four chromosomes. D, Separation of the 
daughter chromosomes formed by longitudional splitting of spireme-thread. E, Beginnings 
of nuclear reconstruction and division of the cell body. F, Cell division complete and 
daughter nuclei in the "resting" stage. 

organized into threads of chromomeres which pair as shown at 5. In this 
diagram the paired threads are taken to represent homologous chromo- 
somes, and the opposite chromomeres in a pair of threads are considered 
as the homologous chromomeres of the two chromosomes. The paired 
threads contract and fuse along their entire length giving the figure 
diagrammed at C in which the two loops represent two pairs of homolo- 
gous chromosomes in the conjugation stage, the essential step in synap- 
sis. Following this stage the two contracted loops of chromatin split 
lengthwise and unravel in somewhat the manner shown in D. These 
filaments contract again forming the intertwined pairs of chromosomes 
shown at E, and the nuclear membrane thereupon begins to disappear. 
Further contraction and the formation of a spindle results in the reduc- 



62 



GENETICS IN RELATION TO AGRICULTURE 



tion figure shown at F, the significant feature of which is the fact that each 
of the daughter nuclei resulting from this division receives only two 
chromosomes instead of the four which the original cell at A contained. 
Since the original cell contained one pair of larger and one pair of smaller 
chromosomes, the daughter cells which are formed each receive one larger 
and one smaller chromosome. 

Cytological investigation is not yet in agreement as to the interpre- 
tation of synapsis especially as to the manner in which the phenomena 
therein concerned are connected with preceding mitotic divisions. Con- 
sidering certain cytological investigations and the results of research in 






Fig. 29. — The reduction division as represented for a species whose diploid number is 
four. A, "Resting" nucleus of a primary germ cell. B, Formaton of paired threads of 
chromomeres. C, Conjugation of homologous chromosomes (synapsis). D, Loosening 
of the synaptic knot. E, Condensation of the chromosomes and disappearance of the nuclear 
membrane. F, Homologous chromosomes about to pass to opposite poles, thus giving each 
secondary germ cell a member of each pair and one-half the somatic number. 

heredity together, it appears that the threads which^'pair in stage B rep- 
resent pairs of chromosomes with homologous chromomeres occupying 
corresponding positions along their entire length. Likewise the contrac- 
tion stage at C is taken to represent a conjugation of the members of pairs 
of chromosomes which later again separate. Other cytological evidence 
indicates that in some forms the conjugation of pairs of homologous chro- 
mosomes is brought about in another way. However, the essential 
fact is the same in either case. In the reduction figure the members of 
each pair of chromosomes are distributed to the opposite poles of the 
spindle so that the daughter nuclei received only one member of each pair. 
The significance of synapsis lies in the conjugation of homologous 



THE PHYSICAL BASIS OF MEN DELI SM 



63 



chromosomes. In the mitoses which have preceded this particular divi- 
sion, the chromosomes were each time conceived to be reformed from the 
identical group of chromomeres which they contained originally. In 
synapsis, however, as shown at B there is a certain amount of intertwin- 
ing of the paired threads and in the unraveling of the chromosomes after 
the contraction stage there is likewise a twisting of the filaments about 
each other. The indications are, therefore, that in synapsis there is a 
possibility of interchange of chromatin material between the members 
of a pair of homologous chromosomes. In all cases, however, in order 
to uphold our conception of the definite organization of the chromosomes 
with respect to the chromomeres which they contain, this interchange of 
material must involve exactly equivalent portions of the two chromo- 



mil" 



Fig. 30. — Diagram of chromatin interchange between homologous members of a pair of 
chromosomes. {After Muller.) 

somes. The chromosomes of the reduction division shown at F may not, 
therefore, be identical with the four originally present in A, but may 
represent various combinations of portions of both members of a par- 
ticular pair of chromosomes. The results of such interchange between 
members of homologous pairs of chromosomes is shown in Fig. 30. At 
the left is shown a pair of chromosomes one in outline the other in full 
black. In the middle the steps in chromatin interchange are diagrammed 
and finally at the right this interchange results in a pair of chromosomes 
each of which is made up of parts of both members of the original pair 
of chromosomes. Various combinations may result depending on the 
points at which interchange takes place, but in every case the exchange 
involves corresponding portions of the two chromosomes. 

Independent Distribution of Chromosomes. — In Fig. 31 are illus- 
trated diagrammatically the chromosomes of Drosophila, with particular 
reference to their size and form relations and to their characteristic 
pairing in the cell. One member of each of these pairs of chromosomes 
was contributed by the female parent and one member by the male parent. 
In the reduction divisions these chromosomes are separated so that 
each germ cell contains one member of each pair of chromosomes. The 
simplest condition which could obtain is that of independent distribu- 



64 GENETICS IN RELATION TO AGRICULTURE 

tion in each pair of chromosomes such that the particular member of one 
pair which went to a given pole of the reduction spindle would have no 
influence on the distribution of the members of any other pair. Such 
independent distribution of chromosomes appears to be actually the type 




m m m m 




Fig. 31. — Diagram showing consequences of independent segregation of chromosomes in 

Drosophila ampelophila. 

followed in reduction. As a consequence the germ cells contain various 
combinations of chromosomes with respect to their original parental deri- 
vation. In Fig. 31 the types of combinations of maternal and paternal 
chromosomes and their mode of derivation in Drosophila are shown 
diagrammatically. Two germ cells, one from the female with the chro- 
mosomes in outline, and the other from the male with the chromosomes 
in full black, unite to form the female zygote shown in the middle of the 
figure. The combinations of maternal and paternal chromosomes which 



THE PHYSICAL BASIS OF MENDELISM 65 

result in the production of germ cells in such an individual are shown 
diagrammatically in the lower portion of the figure. There are eight 
different ways in which the chromosomes may be grouped in the reduc- 
tion figures and on the basis of chance any one of these types is as likely 
to occur as any other. As a result there are sixteen possible combina- 
tions of chromosomes in the germ cells with respect to the original 
derivation of the chromosomes, whether from the female or from the 
male parent. This of course represents only the total number of pos- 
sible combinations of entire chromosomes. By exchange of chromatin 
material between homologous chromosomes resulting in the formation 
of combination- chromosomes the number of actual combinations is 
greatly increased. 

The number of chromosome combinations resulting from independent 
distribution is that number possible when each pair of chromosomes is 
considered separately, and every combination has an equal chance of 
occurrence. With a form having but two pairs of chromosomes there 
would be only four possible combinations, three pairs would give eight, 
four pairs sixteen, and in general the number of possible combinations 
is given by the expression 4" in which n is the number of pairs of chro- 
mosomes in the individual in question. In tobacco which has 24 pairs 
of chromosomes the number of possible combinations in the germ cells 
reaches the enormous total of 16,772,216. This means that in the for- 
mation of zygotes in a self-fertilized tobacco plant the actual parental 
combinations, i.e., combinations identical with those of the germ cells 
which united to form the individual in question, occur only twice in over 
sixteen million times, and this proportion is still further lessened when the 
interchange of chromatin material between homologous chromosomes 
is taken into account. The condition of independent distribution although 
simple in itself results in a rapid increase in complexity with the increase 
in the number of pairs of chromosomes involved. 

Chromosomes and Sex in Drosophila. — The relation between inherit- 
ance and the chromosome mechanism is perhaps most simply displayed 
in the inheritance of sex in those animal forms in which the sexes occur 
in approximately equal proportions. Thus in Drosophila as indicated 
in Fig. 32 there are three pairs of autosomes which are alike in both the 
male and the female. The remaining pair of chromosomes, however, 
differ, for the female possesses two X-chromosomes whereas in the male 
a single X-chromosome is paired with a F-chromosome and these differ- 
ences are characteristic of all normal males and females of this species. 
The bearing of these differences on the inheritance of sex is shown diagram- 
matically in Fig. 32, Beginning with the parents, the diploid number 
is shown in the circles representing the female and the male. 

In the female the three pairs of autosomes are outlined and the X-chro- 



66 



GENETICS IN RELATION TO AGRICULTURE 



mosomes only are drawn in black to indicate that they are the ones pri- 
marily concerned in the determination of sex. Similarly in the male the 
three pairs of autosomes which are exactly like those in the female are 
outlined but the X-chromosome and the F-chromosome are drawn in 




Fig. 32. 



-Diagram to show chromosome relations in the inheritance of sex in Drosophila 
ampelophila. 



black. The reduction division in the female results in a separation of 
the members of each pair of chromosomes, so that every secondary germ 
cell (or egg) contains two large curved autosomes, a small autosome, and 
an X-chromosome. Consequently as far as chromosome content goes 
the eggs are all exactly alike. In the male, however, the separation of 



THE PHYSICAL BASIS OF MEN DELI SM 67 

the members of the chromosome pairs results in sperms half of which 
contain an X-chromosome and half a F-chromosome in addition to the 
three autosomes. The reduction division in the male insures an equality 
in numbers for the two kinds of sperm cells and the chances that either 
kind of sperm will fertilize an egg-cell are equal. By this arrangement the 
numerical equality of the sexes is maintained. When, later, the egg 
cells of the female are fertilized by the sperm cells of the male, as shown 
in the lower portion of the figure, half of them being fertilized by sperm 
cells which contain an X-chromosome will give females, and half uniting 
with sperm cells which contain F-chromosomes will produce males. 
The inheritance of sex in Drosophila provides a beautiful illustration of 
the parallel behavior of the chromosome mechanism and a somatic differ- 
ence, in this case sex. 

To recapitulate, the essential phenomena of cell behavior which fur- 
nish the mechanism for the distribution of hereditary factors are these. 

1. Every species is characterized by a definite number of chromosomes, 
each of which is made up of a definitely organized group of chromomeres. 
The chromosomes occur in pairs, in each of which one member is derived 
from each parent. In ordinary somatic mitosis the distribution of chro- 
matin is such that each daughter cell receives a full complement of chro- 
mosomes which are equivalent qualitatively to those of the mother cell. 

2. In germ cell formation the homologous chromosomes conjugate 
during synapsis, then separate, and pass into a division figure in which 
entire homologous chromosomes are opposed to each other. The re- 
sulting reduction division gives daughter cells with half the number of 
chromosomes characteristic of the species, the half number being made 
up of one member of each pair of chromosomes. During synapsis there 
is an opportunity for the members of a pair of chromosomes to ex- 
change chromatin material. When such interchange takes place equiva- 
lent portions of chromosomes both qualitatively and quantitatively are 
involved. In the reduction division segregation within one pair of chro- 
mosomes is entirely independent of that of any other pair so that the 
combinations of parental chromosomes in the germ cells represent all 
those to be expected on the basis of chance distribution. 

The student should constantly endeavor to harmonize this conception 
of the distributing mechanism of the chromatin material with the Men- 
delian interpretations of hereditary phenomena which will be presented 
in what follows, to the end that he may obtain a clear and definite idea of 
the interrelations between the known facts of heredity and cell behavior. 



CHAPTER V 
INDEPENDENT MENDELIAN INHERITANCE 

Essentially Mendelism is an attempt to explain the result of heredity 
on a rigid, statistical basis. Morgan has stated that the cardinal feature 
of Mendelism is the fact that when the hybrid forms germ cells the 
factors segregate from each other without having been contaminated one 
by the other. The presence or absence of any contamination of factors 
is still a debatable subject as will be apparent from later discussions, but 
for all practical purposes the absence of such contamination may be re- 
garded as an established fact. The other implications of this statement 
that the two germ-cells which unite to produce the individual each con- 
tribute an homologous set of hereditary units or factors which determine 
the characters of the individual and that these units again separate from 
each other in germ-cell formation are the fundamental conceptions of 
Mendelism. When the units are considered pair by pair one member of 
each of which has been derived from each parent, it is clear that the im- 
portant feature of Mendel's discovery lies in the segregation of the 
members of each pair in germ-cell formation. 

The statistical laws of segregation of characters were first announced 
by Johann Gregor Mendel, Augustinian monk and later Pralat of the 
Konigskloster at Briinn, Austria. In 1865 after 8 years of thorough 
and painstaking research which is even today a model of genetic inves- 
tigation, he read the results of his investigations before a meeting of a 
local scientific society, the Natural History Society of Briinn, and the 
following year the paper was published in the transactions of this society. 
Unfortunately, however, the announcement of the work was made at a 
time when the scientific world was not in a position to appreciate its 
full significance and was busy with other things. The results, therefore, 
were neglected until in 1900, the independent investigations by the 
three botanists, Correns, von Tschermak, and de Vries, led to similar 
conclusions and to the rediscovery of Mendel's paper. By that time 
experimental research had so far advanced that the importance of Mendel's 
work was immediately recognized and it was not long before a vast 
series of investigations had been reported in confirmation of it. 

The Monohybrid. — The operation of Mendelism is best followed by 
considering an actual experiment. Mendel crossed tall and dwarf peas 
and obtained hybrid plants, all of which were tall like the tall parent. 

68 



INDEPENDENT MENDELIAN INHERITANCE 69 

When the progeny of these tall hybrid plants were grown three-fourths 
of the plants were tall, like the original tall variety, and one-fourth were 
dwarf, like the original dwarf variety. Although like the tall plant in 
appearance, therefore, the tall hybrid plants which were produced by 
crossing a tall and a dwarf plant displayed their hybrid nature in the 
kind of progeny they produced. To distinguish them from the tall 
parents which produced only tall plants, they are accordingly called 
tall hybrids. Continuing this experiment, Mendel found that the dwarf 
segregants of the second generation bred true, they produced only 
dwarf plants; but of the tall plants one-third only bred true, and the 
other two-thirds proved to be tall hybrids, for three-fourths of their 
progeny were tall plants and one-fourth dwarfs. The progeny of the 
original tall hybrid plants, therefore, when subjected to this analysis 
was found to consist of 1 tall : 2 tall hybrid : 1 dwarf. The experimental 
results of the hybridization of tall and dwarf peas may accordingly be 
diagrammed as in Fig. 33. 

Tall X • Dwarf 

1 
Tall hybrids 



1 Tall 2 Tall hybrid 1 Dwarf 

I . ■ . I 

Tall 1 Tall 2 Tall hybrid 1 Dwarf Dwarf 



i i 

Tall Tall 1 Tall : 2 Tall hybrid : 1 Dwarf Dwarf Dwarf 

Fig. 33. — Results of hybridization of tall and dwarf peas. 

Mendel studied hybrids involving several different pairs of contrasted 
characters and found that in every case one member of each pair of 
characters was expressed unchanged in the hybrids, whereas the other 
member of the pair became latent and its presence could be detected only 
by growing the progeny of the hybrid. Those characters which were 
expressed unchanged in the hybrid Mendel termed dominant, the latent 
characters he called recessive. In the above experiment, for example, 
tallness was dominant and dwarfness, recessive. Mendel saw that the 
dominant character, therefore, in these experiments possessed a double 
significance, that of parental character in which case a uniform progeny 
of dominants is produced and that of a hybrid character in which case 
one-fourth of the offspring display the contrasted recessive character. 
In the above experiment the parental dominants are the tall parents and 
the hybrid dominants are the tall hybrids. The condition of dominance 
for a character, therefore, is determined by the fact that in the hybrid 
that character is expressed to the exclusion of its contrasted character. 
Dominance is by no means a universal phenomenon, but in Mendel's 



70 



GENETICS IN RELATION TO AGRICULTURE 



T 






T 




t 






t 






T 


T 




t 


t 




11 II 1 


T 






T 




t 






t 



experiments it so happened that one member of each of the seven pairs 

of characters displayed complete dominance. 

The explanation for the appearance of the recessive character in 

the second generation and in subsequent generations rests on the fact 

that the contrasted characters are 
represented in the germ cells by units 
or factors. The factor for tallness 
may be represented by T and the 
factor for the contrasted character 
dwarfness by t. The relations which 
exist when plants bearing these dif- 
ferent factors are crossed are shown 
in Fig. 34. In the tall race of plants 
the gametes all bear the factor T, 
so that since any individual of this 
race arises from the union of two 
germ cells its genetic constitution 
with respect to this particular factor 
is TT. Similarly the genetic con- 
stitution of any plant of the dwarf 
race is represented by tt and it pro- 
duces germ cells each of which bears 
the factor t. When tall and dwarf 
plants are crossed, the hybrid receives 
a factor T from one germ cell and a 
factor t from the other, so that the 
tall hybrids which are produced are of 
the genetic constitution Tt. 

In the production of germ cells 
and in the union of these germ cells 
to produce the individuals of the 
second generation is seen the opera- 
tion of Mendelian principles. The 
contrasted units T and t separate in 
the germ cells of the offspring so that 

a particular germ cell receives only one of these factors, either T or t. 

Half the germ cells consequently bear the factor T and half bear the 

factor t, and this is true of both pollen grains and ovules. 
When a tall hybrid plant is self -fertilized, therefore: 

a T ovule may be fertilized by a T pollen grain producing a TT plant, tall, 
a T ovule may be fertilized by a t pollen grain producing a Tt plant, tall hybrid, 
a t ovule may be fertilized by a T pollen grain producing a Tt plant, tall hybrid, 
a t ovule may be fertilized by a t pollen grain producing a tt plant, dwarf. 




Ovules. 





/ 










/ 


T 


T 






T 


t 




'' 


TALL 




T/ 


VLLHYBR 


D 
















T 


t 






t 


t 




V 


VLL HYBf 


<ID 




DWARF 





Fig. 34. — Diagram showing factor history 
in a cross between tall and dwarf peas. 



INDEPENDENT MEN DELI AN INHERITANCE 71 

Since there is an equal chance for the occurrence of any one of these 
types of combinations the progeny of a tall hybrid plant are in the ratio 3 
tall : 1 dwarf. One-third of the tall plants are of the genetic constitu- 
tion TT and they consequently will produce only tall plants, whereas the 
other two-thirds are of the genetic constitution Tt and will display segrega- 
tion in the ratio 3 tall : 1 dwarf. The dwarfs are all of the genetic constitu- 
tion tt, consequently they can produce only dwarf plants. The explana- 
tion, therefore, satisfies all the requirements laid down by the experimental 
results. 

Mendelian Terminology. — As a result of the rapid development of 
Mendelism during the past few years, a special terminology has grown 
up which is used by practically all investigators in heredity. For those 
terms which are in most common use, the following brief statements 
are intended as interpretations of meanings and significance rather than 
as mere definitions. 

The germinal representatives of Mendelian characters are variously 
termed genes, factors, or determiners, three terms which are used synony- 
mously in Mendelian literature. A Mendelian factor may be defined as 
an independently inheritable element of the genotype by the presence 
of which the development of some particular character in the organism 
is made possible. The word gene was introduced by Johannsen to 
designate an internal condition or element of the hereditary material upon 
which some morphological or physiological condition of the organism is de- 
pendent. These definitions do not hold rigidly as is always the case with 
attempts to define something about which very little is known. Of the 
terms, the teiin gene as introduced by Johannsen expressly denies any 
assumptions as to the ultimate nature of the unit in question. The word 
determiner on the other hand since it implies a rigid relation between an 
hereditary unit and its end product, the character, is falling into disre- 
pute, for very probably many hereditary units are concerned in the pro- 
duction of all characters. The term factor, as applied to the units of 
Mendelian heredity is perhaps more frequently used than any other and 
is just as free from undesirable implications as to the nature of these 
units on the one hand or their relation to the characters of the individual 
on the other hand. It will consequently be used more frequently in this 
book. 

Unit characters are those characters of the individual which behave 
as units in heredity. Thus tallness and dwarfness in peas, since they 
behave as units in heredity are called unit characters. To behave strictly 
as units in heredity, character contrasts must depend on single factor 
contrasts, as for example the character contrast of tall vs. dwarf in peas 
depends upon a contrast of the factors T and t. The term is a survival 
of the early days of MendeHsm when attention was focussed on the 



72 GENETICS IN RELATION TO AGRICULTURE 

character rather than on the factor as is today the case; and we now have 
numerous examples of characters which behave as units in certain con- 
trasts, but in others behave as compound characters. It is, therefore, 
questionable whether in a rigid sense there are any such things as unit 
characters, but the term has been much used in Mendelian literature, 
and the conception to which it gives rise, namely that particular indi- 
viduals or races possess a number of unit characters which may be dis- 
sociated from them and recombined in various fashions with the unit 
characters of related individuals or races, is a useful one and is strictly 
in accordance with experimental results. 

Allelomorphs are contrasted factors or characters. More rigidly as 
applied to characters, an allelomorph is one of a pair of charactel-s which 
display alternative inheritance, i.e., inheritance in which one or both of 
the contrasted characters, although obscured, retain their identity and 
emerge unchanged from the hybrid. With respect to factors allelo- 
morphism is a relation between two factors such that they are sepa- 
rated into sister gametes in germ-cell formation; they never both enter 
the same gamete. The allelomorphic characters in our sample are 
characters tallness and dwarfness, and correspondingly the factors T and 
t are allelomorphs. 

The genotype is the constitution of an organism with respect to the 
factors of which it is made up. Rigidly the genotype is the sum total of 
genes or factors of an individual, but it is customary to speak of the sum 
total of analyzed factors which are under immediate consideration as the 
genotype. The genotype of the tall race of peas in the above experiment 
was TT, of the dwarf race tt. The factor arrangement of an individual 
is also called its genetic constitution when a particular set of factors are 
concerned and this term is also employed to designate a particular set 
of factors carried by a gamete. Genotypes of the constitution TT or tt, 
or in general those which receive the same factors from both gametes 
are homozygous, whereas those which receive different factors from the 
two germ cells or gametes are heterozygous, as for example plants of 
the genetic constitution Tt. Similarly an individual contains a duplex 
dose of a given factor when it receives that factor from both parents, or 
a simplex dose if the factor comes in in only one of the germ cells. The 
substantives corresponding to the adjectives homozygous and hetero- 
zygous are homozygote and heterozygote, respectively. 

The phenotype is the aggregate of the externally obvious characters of 
an individual or a group of individuals. Thus in the second generation of 
the above, experiment there were two phenotypes, tall and dwarf, and all 
the second generation plants belonged to one or the other of these classes. 
Moreover all members of a phenotype do not necessarily possess the 
same genetic constitution. In the above example the tall phenotype 



INDEPENDENT MENDELIAN INHERITANCE 73 

included tall plants of the genetic constitution TT and tall hybrids of 
the genetic constitution Tt. The distinction between the genotypes of a 
given phenotype is only possible by further breeding tests. In general 
a hybrid is best detected by crossing it to the recessive form in which 
case it will produce half dominants and half recessives, whereas the pure 
dominant will produce only dominants. Such a cross is known as a back 
cross or sesqui-hybrid. 

With respect to history an extracted dominant or recessive is one 
which has been derived from a hybrid form. The historical fact with re- 
gard to an extracted form that the parent or other known ancestor did not 
breed true for the character in question is the only distinguishing feature 
about it, the factors which it contains are the same as those in the parent 
races. 

The parents of a hybrid are generally called the Pi generation. The 
progeny obtained by crossing two distinct races is the first filial genera- 
tion, conveniently designated the Fi. The progeny of the Fy are the F2 
generation and so on. 

The above terms are constantly employed in even the most simple 
cases and their application will soon become clear to the student. Other 
terms are used in connection with more complex cases, but these will be 
introduced only when their significance may be made clear from the 
manner in which they are employed. 

The Chromosome Interpretation. — The chromosome interpretation 
of a case of monohybridism is very simple. It depends on the assumption 
for the case of tall vs. dwarf peas that the factor IT is a chromomere 
occupying a definite position in each member of a certain pair of 
chromosomes of the tall race. The factor t is correspondingly located 
in exactly the same pair of chromosomes in the dwarf race. Aside 
from this difference in one pair of chromomeres which occupy identical 
positions in corresponding pairs of chromosomes the chromosomes of 
the two races bear exactly the same set of factors. Accordingly, of the 
seven pairs of chromosomes in the cells of the garden pea, only that pair 
need be considered which bears the factor 7", or in the dwarf races its 
allelomorph, the factor t. In the hybrid produced by crossing a tall and 
a dwarf pea one member of the pair of chromosomes bears the factor T, 
and the other the factor t. In the reduction divisions the members of 
this pair of chromosomes are separated and distributed to different germ 
cells, consequently half the number of germ cells will receive that mem- 
ber which bears the factor T, and half that member which bears the factor 
t. Recombination of these gametes gives the offspring in the ratio 3 
tall : 1 dwarf, which has been pointed out previously. If in Fig. 34 the 
rectangles containing the factors T and t are taken to represent the mem- 
bers of this pair of chromosomes instead of entire gametes, this figure 



74 GENETICS IN RELATION TO AGRICULTURE 

may be used to illustrate the history of this pair of chromosomes in hy- 
bridization. Since chromosome relations have been determined more 
definitely in Drosophila, we shall follow out in detail a selected case in 
this species. We have pointed out previously that aside from the pair 
of sex-chromosomes, the pairs of chromosomes in both the male and fe- 
male of Drosophila are alike and bear the same factors. But in the male 
the F-chromosome appears to have no effect upon the development of 
the body characters so that the male depends upon a single X-chromo- 
some for the development of those characters determined by the factors 
borne in this chromosome. The F-chromosome may, therefore, be re- 
garded as a neutral mate for the X-chromosome in the male. Since the 
distribution of this pair of chromosomes is unique as we have pointed out 
in the discussion of the inheritance of sex in Drosophila, the history of 
factors carried by the X-chromosomes furnishes a beautiful illustration 
of the parallelism existing between chromosome behavior and factor 
distribution. The inheritance of white-eye color in Drosophila is a case 
in point. Wild races of Drosophila have red eyes, but Morgan discovered 
a few white-eyed male mutants in an inbred strain of "wild" flies, i.e., 
flies which were directly descended from wild flies. From this muta- 
tion it was found possible to establish a white-eyed race of flies which 
breed true to this new eye character. When a white-eyed male is 
mated to a red-eyed female the offspring all have red eyes, because 
red eye in Drosophila is dominant to white. In F2 red- and white- 
eyed flies are produced in the proportion of 3 red : 1 white. All the fe- 
males in this generation are red-eyed, but of the males half have red and 
half white eyes. When the reciprocal cross is made, i.e., when a white- 
eyed female is mated to a red-eyed male the results are different. In 
the Fi of such a mating the female flies have red eyes and the males 
all have white eyes. When the Fi flies are bred together an F2 is obtained 
half the females of which have red eyes and half white eyes, and likewise 
among the males half have red eyes and half white eyes. 

The explanation of this type of inheritance is shown diagrammatically 
in Figs. 35 and 36. The factor for white eyes is represented by w and it 
is borne in the X-chromosome. The factor W for red eyes, allelomorphic 
to w, is carried by the X-chromosome of the red-eyed race of flies at ex- 
actly the same locus as that of w in the white-eyed race. Since these 
two factors occupy the same locus in the X-chromosome obviously they 
can never be contained in the same chromosome. In Fig. 35, the two 
X-chromosomes of the red-eyed female both contain the factor W for 
red eyes. In a convenient shorthand system the genetic constitution 
for such a fly may be designated {WX)(WX), the parenthesis indicating 
that the factor W is carried by the X-chromosome. Each egg from such 
a female will contain an X-chromosome with a factor for red eyes — in 



INDEPENDENT MENDELIAN INHERITANCE 



75 



our shorthand notation they will all be (WX). On the other hand, 
the white-eyed male will produce sperm cells half of which have an X- 
chromosome and half a 1^-chromosomc. The X-chromosomc of the sperm 
cells carries a factor id for white eyes, but the F-chromosome does not bear 




Fig. 35.^ — Inheritance of white eye color in Drosophila. Red- eyed female mated to 
white-eyed male. Solid lines indicate history of chromosomes of female; dotted and 
gray lines, of the male. (Adapted from Morgan.) 

this factor. In the shorthand notation these two kinds of sperm are rep- 
resented by {wX) and Y respectively. The F-chromosome is drawn in 
black to indicate its unknown constitution with respect to the factors it 
contains. When an egg-cell (WX) of the red-eyed female is fertilized 
by a (wX) sperm cell, a female is produced of the genetic constitution 



76 



GENETICS IN RELATION TO AGRICULTURE 



(WX){wX), and it will be red-eyed because red is dominant over white. 
When such an egg is fertilized by a F-bearing sperm a male is produced of the 
genetic constitution {WX)Y and it is red-eyed because the X-chromosome 
of the egg-cell carries the factor W. In the Fi female the reduction divi- 




FiG. 36. — Inheritance of white eye color in Drosophila. White-eyed female mated with 
red-eyed male. Dotted lines indicate history of chromosomes of female; solid and gray 
lines, of the male. (Adapted from Morgan.) 

sions separate the X-chromosome bearing the factor for red eyes from 
the X-chromosome bearing the factor for white eyes. Consequently the 
Fi female produces two types of eggs, (WX) and (ivX). In the Fi male 
the sperm cells similarly produced will be of two kinds (TFX) and Y. 
As shown in the diagram there are four possible combinations of such 



INDEPENDENT MENDELIAN INHERITANCE 77 

egg and sperm cells in F^ and these give red-eyed females half of which 
are homozygous {WX){WX) and half heterozygous {WX){wX) and 
equal numbers of red-eyed and white-eyed males {WX)Y and {wX)Y 
respectively. 

In the reciprocal cross, Fig. 36, the white-eyed female contains two 
X-chromosomes each bearing a factor for white eyes. Her genetic 
constitution, therefore, is {wX){wX). All the eggs from such a female 
will be of the genetic constitution {wX) — they contain an X-chromosome 
bearing a white-eye factor. When such eggs are fertilized by an X- 
bcaring sperm cell from the male, the female produced will be of the 
genetic constitution {WX){'wX). It will be red-eyed because of the red- 
eyed factor carried by the X-chromosome of the sperm. On the other 
hand, when such an egg is fertilized by a F-bearing sperm cell, the male 
thus produced will be of the genetic constitution {wX)Y. It will be 
white-eyed, because of the white-eye factor in the X-chromosome of the 
egg-cell. Breeding two such Fi individuals together will result in the 
Fi distribution shown in the diagram. Females will be produced half 
of which are of the genetic constitution {WX){wX) and half {wX){wX), 
hence red-eyed and white-eyed respectively; and males half of the genetic 
constitution {WX)Y and half {wX)Y, hence red-eyed and white-eyed 
respectively. The peculiar relations exhibited in the inheritance of 
white-eye color in Drosophila, therefore, admit of a logical chromosome 
interpretation, if we assume that the factors involved are borne by the 
X-chromosomes. The type of inheritance which is apparently dependent 
on factors borne in the sex-chromosomes is called sex-linked inheritance. 
It will be treated more fully in Chapter XI. 

Mathematical Adequacy of Mendelism. — Mendelian principles do 
not apply to isolated phenomena of inheritance alone, but they are of 
general significance. It is consequently of interest to know how well 
experimental results agree with theoretical expectations when Mendel- 
ian analyses are rigidly applied. Particularly is this true of the mathe- 
matical relations involved, which have often been used to confute the 
arguments of Mendelian interpretations. We shall accordingly con- 
sider the results of Mendel's original investigation from this standpoint, 
and a few other cases which have been investigated in particularly large 
progenies and under circumstances which practically eliminate personal 
bias. 

Mendel's investigations with peas included a consideration of seven 
pairs of contrasted characters as follows: 

1. The Difference in Form of Ripe Seeds. — These are either round 
or roundish, the depressions, if any, occur on the surface, and are at most 
only shallow as in the indent type; or they are irregularly angular and 
deeply wrinkled. 



78 



GENETICS IN RELATION TO AGRICULTURE 



2. The Difference in Color of the Cotyledons. — The cotyledons of the 
ripe seeds are either pale yellow, bright yellow, or orange-colored, or 
they possess a more or less intense green tint. 

3. The Difference in Color of the Seed Coat. — This is either white, with 
which character white flowers are constantly correlated; or it is gray, 
gray-brown, leather brown, with or without violet spotting, in which 
case the color of the standards is violet, that of the wings purple, and 
the stem at the base of the leaves is of a reddish tint. 

4. The Difference in Form of the Ripe Pods. — These are either 
simply inflated, not contracted in places; or they are deeply constricted 
between the seeds and more or less wrinkled. 

5. The Difference in Color of the Unripe Pods. — They are either light 
to dark green, or vividly yellow, in which coloring the stalks, leaf veins, 
and calyx participate. 

6. The Difference in Position of the Flowers. — They are either axillary, 
that is distributed along the main stem; or they are terminal, that is 
bunched at the top of the stem and arranged almost in a false umbel; 
in this case the upper part of the stem is more or less widened in cross- 
section. 

7. The Difference in Length of the Stem. — The length of the stem is 
very various in some forms; it is, however, a constant character for 
each, in so far that healthy plants, grown in the same soil, are only sub- 
ject to unimportant variations. In the experiments a long axis of 6 to 7 
feet was always crossed with a short one of ^ to 13^^ feet. 

The results of segregation in F2 in these seven series of experiments 
have been summarized from Mendel's paper in Table VII: 

Table VII.— Summary of Mendel's Experiments with Peas 



No. 


Character contrast 


No. in F2 


Dominants 


Recessives 


Ratio per 4 


1 


Form of seed 


7,324 

8,023 
929 

1,181 
580 
858 

1,064 


5,474 
6,022 
705 
882 
428 
651 
787 


1,850 
2,001 
224 
299 
152 
207 
277 


2.99 :1.01 


2 
3 
4 


Color of cotyledons 

Color of seed coats 

Form of pod 


3.00:1.00 
3.04:0.96 
2.99:1.01 


5 


Color of pod 


2.95 :1.05 


6 

7 


Position of flowers 

Length of stem 


3.03:0.97 
2.92 :1.08 




Totals 


19,959 


14,949 


5,010 


2.996 : 1.004 









Mendel observed no transitional forms in these experiments so that 
the ratios he obtained are based entirely on unprejudiced observations. 
The ratios in no case differ significantly from the ideal 3:1 ratio. Several 
investigations have furnished confirmatory evidence as to the correctness 



INDEPENDENT MEN DELI AN INHERITANCE 



79 



of these observations, particularly with respect to that pair of characters 
concerned with cotyledon color. Johannsen has summarized these 
results and examined them with reference to their agreement with the 
conditions imposed by the laws of chance. Table VIII which has been 
adapted from Johannsen shows that in a sum total of 179,399 counts 
by seven different investigators the ratio was 3.0035:0.9965. The 
probable error for this number of observations is ± 0.0028 so that the 
deviation from the ideal ratio is slightly greater than the probable error, 
but only so great that such a deviation would be expected approximately 
twice in five times. 

Another case which has been investigated with very large numbers 
is that of the contrasted characters starchy and sweet endosperm in 




. f *f *f' f >»» | »! f < l f» | <t» |ii tt § §i 



Fig. 37. — Results of crossing starchy and sweet corn : a, Sweet parent ; c, starchy parent ; 
b, the Ft showing complete dominance ^of starchiness; d, the F2 showing monohybrid 
segregation; e, f, g, and h, Fz populations, the last three obtained by planting F2 starchy 
grains, the sweet ear, e, by planting an F2 sweet grain. {After East and Hayes.) 

maize. Those varieties of maize which have starchy endosperms have 
smooth opaque grains whereas the varieties with sweet endosperms have 
translucent, wrinkled grains. The difference is due to the fact that in 
ripening there is a progressive formation of starch in starchy races, but 
in sweet races the starch grains formed are small and angular and there 
is an actual breaking down of endosperm materials into various kinds 
of sugars. Correns has shown that starchiness is completely dominant 
and segregation is sharp and unquestionable aside from very exceptional 
cases of intergrading. Fig. 37 illustrates very welt how sharply segre- 
gation occurs in hybrid ears. The results of East and Hayes' extensive 
investigations of segregation for this pair of characters are summarized 
in Table IX. In this table famihes have been entered separately so that 



80 



GENETICS IN RELATION TO AGRICULTURE 



the close correspondence to expectation can be seen all along the line. 
Moreover, these families represent crosses between many different 
starchy and sweet races, so that the observations are not based on a 
single hybrid. The deviation of the total ratio from the 3 : 1 ratio is 
very slight and falls far within the probable error set by mathematical 
conditions. 



Table VIII. — Summary of Investigations on Inheritance of Cotyledon Color in 

Pi SUM (After Johannsen) 



Investigator 


Yellow 


Green 


Total 


Ratio per 4 


Probable errors 


Mendel, 1865. . . 


6,022 


2,001 


8,023 


3.0024 : 0.9976 


±0.0130 


Correns, 1900.. 


1,394 


453 


1,847 


3.0189 :0.9811 


±0.0272 


Tschermak,1900 


3,580 


1,190 


4,770 


3.0021 : 0.9979 


±0.0169 


Hurst, 1904.... 


1,310 


445 


1,755 


2.9858 : 1.0142 


±0.0279 


Bateson, 1905 . . 


11,902 


3,903 


15,806 


3.0123 : 0.9877 


±0.0093 


Lock, 1905 


1,438 


514 


1,952 


2.9467 : 1.0533 


±0.0264 


Darbishire, 1909 


109,060 


36,186 


145,246 


3.0035 : 0.9965 


±0.0030 


Totals 


134,707 


44,692 


179,399 


3.0035 : 0.9965 


±0.0028 



Table IX. — Segregation of Starchy vs. Sweet Endosperm in Maize 



Family No. 


Starchy 


Sweet 


Total 


Ratio 


(15 X 54) 


1,746 


623 


2,369 


2.948:1.052 


(15 X 54) - 2 


2,293 


728 


3,021 


3.036:0.964 


(24 X 54) 


2,288 


801 


3,089 


2.963:1.037 


(24 X 54) - 1 


771 


269 


1,040 


2.965:1.035 


( 5 X 18) 


1,509 


492 


2,001 


3.017:0.983 


(11 X 18) 


873 


319 


1,192 


2.930:1.070 


(17 X 54) - 1 


328 


102 


430 


3.051:0.949 


(18 X 58) - 1 


332 


102 


434 


3.060:0.940 


( 7 X 54) 


872 


268 


1,140 


3.060:0.940 


( 8 X 54) 


1,505 


530 


2,035 


2.958:1.042 


( 8 X 54) - 1 


3,524 


1,163 


4,687 


3.008:0.992 


( 8 X 54) - 5 


2,190 


725 


2,915 


3.005:0.995 


(19 X 7) 


783 


230 


1,013 


3.092:0.908 


(19 X 7) - 5 


304 


109 


413 


2.944:1.056 


(19 X 8) 


1,813 


602 


2,415 


3.003:0.997 


(60 - 5 X 54) 


1,150 


379 


1,529 


3.009:0.991 


(60 - 3 X 54) 


799 


267 


1,066 


2.998:1.002 


(60 - 8 X 54) 


451 


138 


589 


3.063:0.937 


Totals 


23,531 


7,847 


31,378 


2 . 9997 : 1 . 0003 






Probable error 


±0.0062 



INDEPENDENT MENDELIAN INHERITANCE 



81 



These two cases illustrate very well how closely the results of Men- 
delian investigations fulfill mathematical requirements, and their signifi- 
cance cannot be doubted when it is considered how little difficulty is 
experienced in classifying this particular kind of material. Nevertheless 
the mathematical requirements are very often not fulfilled on account 
of the action of external conditions of various kinds. Here as elsewhere 
the disturbing influence of biological factors must ever be kept in mind 
in judging the significance of the application of any strict mathematical 
tests. 

Dihybridism. — When two pairs of factor differences are involved in 
a hybrid the same laws apply in segregation and recombination as apply 
in the monohybrid. The two pairs of factors segregate independently 




Fig. 38. — Maize ear showing F2 segregation of grains in the ratio of 3 purple, 1 white. 



of each other and give character combinations in F2 to be expected on the 
basis of chance factor distribution. In maize there are varieties which 
have a deep purple aleurone color which gives the entire grain a black 
appearance. When such varieties are crossed with certain white varie- 
ties which possess no aleurone color the Fi is purple and in F2 the grains 
are in the ratio of 3 purple : 1 white. An ear displaying such F2 segrega- 
tion is shown in Fig. 38. The factors involved in this case are W for 
pigment production in the aleurone layer and w for no pigment pro- 
duction in this tissue. The hybrid Ww since it is a monohybrid will, 
therefore, give in F2 genotypes in the ratio 1 WW^ : 2 Ww : 1 ww, which are 
distributed in two phenotypes in the ratio 3 purple : 1 white. We have 
shown similarly how starchy corn when crossed with sweet gives a starchy 
Fi and in F2 3 starchy : 1 sweet. Here the factors involved are S for 
starchiness and s for sweet. A purple sweet corn, therefore, will have 
the genetic constitution TFPFss with respect to the above factors, and a 
white starchy corn, the genetic constitution wwSS. 

When a purple sweet corn is crossed with a white starchy corn the 
Fi will be purple starchy — it will display the dominant characters of both 
parents to the exclusion of the recessive characters, white and sweet. 
From the purple sweet corn the Fi receives gametes of the genetic consti- 
tution Ws and from the white starchy wS. Consequently its genetic 



82 GENETICS IN RELATION TO AGRICULTURE 

constitution is WwSs, and it contains two pairs of factor contrasts. 
Such a hybrid produces gametes representing all possible combinations 
containing one member of each pair of factors. The gametes, therefore, 
will be produced in the combinations and proportions 

1 WS:1 Ws:l wS:l ws. 

This series of gametes will be represented in both the pollen grains and 
ovules, so that if each kind of ovule has an equal chance of being fertil- 
ized by any one of the four kinds of pollen grains the following combina- 
tions will result. 

Table X. — Combinations of Factors and Characters Resulting from 

Self-fertilization op a Purple Starchy Corn of the 

Composition WwSs. 

WS ovule and WS pollen grain gives WWSS, purple starchy 
WS ovule and Ws pollen grain gives WWSs, purple starchy 
WS ovule and wS pollen grain gives WwSS, purple starchy 
WS ovule and ws pollen grain gives WwSs, purple starchy 

Ws ovule and WS pollen grain gives WWSs, purple starchy 

Ws ovule and Ws pollen grain gives TFWss, purple sweet 

Ws ovule and wS pollen grain gives WwSs, purple starchy 

Ws ovule and ws pollen grain gives Wwss, purple sweet 

wS ovule and WS pollen grain gives WwSS, purple starchy 

wS ovule and Ws pollen grain gives WwSs, purple starchy 

wS ovule and wS pollen grain gives ivwSS, white starchy 

wS ovule and los pollen grain gives wwSs, white starchy 

ws ovule and WS pollen grain gives WwSs, purple starchy 

ivs ovule and Ws pollen grain gives Wwss, purple sweet 

ws ovule and wS pollen grain gives ivwSs, white starchy 

ws ovule and ws pollen grain gives wwss, white sweet 



When the F2 grains are classified according to their phenotype, 
they are distributed as follows: 

9 grains with purple aleurone and starchy endosperm 
3 grains with purple aleurone and sweet endosperm 
3 grains with white aleurone and starchy endosperm 
1 grain with ivhite aleurone and sweet endosperm 

Just as the 3 : 1 ratio is typical for the monohybrid when one of the 
contrasted characters is dominant, so the 9:3:3:1 ratio is charac- 
teristic of dihybrids when one member of each pair of characters is domi- 
nant. This ratio is clearly derivable from the simple 3 : 1 ratio, for 
considering first aleurone color, the segregation is in the ratio 3 purple : 1 



INDEPENDENT MEN DELI AN INHERITANCE 83 

white. When the endosperm segregation into starchy and sweet is taken 
into account in the same hybrid the segregation will be in the ratio of 
3 starchy : 1 sweet in each of these classes, for these characters segregate 
independently of the aleurone color. This gives, therefore, 3 purple 
(3 starchy : 1 sweet) : 1 white (3 starchy : 1 sweet) which becomes on 
expansion 9 purple starchy : 3 purple sweet : 3 white starchy : 1 white 
sweet. 

The correlation of the above facts with chromosome behavior is 
again very simple. The factors W and w lie in identical positions in one 
pair of chromosomes and the factors S and s lie in identical positions in a 
different pair of chromosomes. If the difference between the two varie- 
ties of maize is only in these factors, then all the other pairs of chro- 
mosomes in the varieties bear the same set of factors. Accordingly of 
the ten pairs of chromosomes of maize only those two need be considered 








Fig. 39. — -Chromosome behavior in reduction in Fi from a cross between purple 
sweet and white ;starchy corn. Factor symbols: w = white, PF = purple, s = sweet, 
S = starchy. 

which bear the above factors. The relations then are shown diagrammat- 
ically in Fig. 39. The parents in both cases produce gametes which are 
all alike. The crossing of these parents produces a zygote in which two 
pairs of the chromosomes differ in their factor content. One member 
of one pair bears W and the other w and in the other pair one member 
bears S and the other s. Two types of Fi reduction division are 
possible and these give four kinds of gametes as shown in the diagram. 
Since this has occurred in the formation of both ovules and pollen grains, 
in the self-fertilization of such a plant there are sixteen possible combina- 
tions of gametes, which distribute themselves in four phenotypes in the 
ratio 9 purple starchy: 3 purple sweet: 3 white starchy:! white sweet. 
This feature of the case has already been discussed fully and need not be 
repeated here. 

The actual agreement of this analysis with experimental results 
has been shown by several investigators but particularly by East and 
Hayes. In one case they crossed a white flint corn, Rhode Island White 
Cap, with a purple sweet corn. Black Mexican. The Fi grains were 
purple starchy and in F2 there was sharp segregation for purple and white 
aleurone and starchy and sweet endosperm. In some cases splashed 



84 



GENETICS IN RELATION TO AGRICULTURE 



Table XI. — Class Frequencies Among Grains From Fi and Fz Ears From 
THE Cross Purple Sweet X White Starchy {From East and Hayes) 



Generation 


Ear No. 


Purple 
starchy 


Purple 
sweet 


White 
starchy 


White 
sweet 


Total 


F, 


(24 X 54)-l 
(24 X 54)-2 
(24 X 54)-6 
(24 X 54)-8 
(24 X 54)-10 
(24 X 54)-ll 
(24 X 54)-13 


207 

307 

170 

164 

197 

194 

159 

148 

166 

152 

166 
163 

205 

208 


67 
69 

54 
55 

65 
65 

41 
60 

40 
51 

55 

54 

81 
69 


67 
69 

49 

55 

59 
65 

41 

50 

46 
61 

47 
64 

59 
69 


27 
'23 

19 
18 

24 

22 

23 

16 

19 
17 

22 
18 

25 

23 


368 
292 
345 
264 
271 
290 
370 


Fa 


(24 X 54)-l-2 

(24 X 54)-l-6 

(24 X 54)-l-8 

(24 X 54)-l-9 
1 


161 

155 

171 

168 

180 
183 

79 
80 


55 

52 

56 
56 

71 
61 

29 

27 


46 

52 

52 
56 

55 
61 

27 
27 


13 

17 

19 
19 

19 

20 

7 
9 


275 
298 
325 
142 . 


F2 


Totals 


1,270 
1,237 


403 
413 


368 
413 


159 

137 


2,200 


Fa 


Totals 


591 
585 


211 

195 


180 
195 


58 
65 


1,040 


Combined Totals 




1,861 

1,822 


614 
608 


548 
608 


217 

202 


3,240 









INDEPENDENT MENDELIAN INHERITANCE 



85 



purple grains were obtained, but further breeding tests showed that 
these were simply heterozygous for purple coloration. A real exception 
must, however, be made for certain families which showed aleurone 
color segregation in the ratio 9 purple:? white. Such results depend 
on the presence of two color factor differences and they will be explained 
later. The results in F2 and F3 for these plants of the genetic consti- 
tution WwSs are tabulated in Table XI . The expected results in each case 
are given in italics. 

Throughout the results in this table are substantially in agreement 
with theoretical requirements. The hypothesis has, however, been sub- 
jected to the further test of growing F3 populations. Table XII shows the 
kind of Fz populations which are to be expected when F2 grains from 
this cross are planted. All these types of populations were secured. The 
case, therefore, provides an excellent illustration of the way in which a 
Mendehan experiment is carried out and of the excellent agreement 
with theory which is given in such experiments. 

Table XII. — Fa Ratios to be Expected from the Different Genotypes in the 

Cross WWss X wwSS 





Genotype 


Ratio in Fa 


Phenotype 


Purple 
starchy 


Purple 
sweet 


White 
starchy 


White 
sweet 


Purple starchy 


WWSS 
WWSs 

WwSS 


All 
3 


1 


1 

3 






WwSs 9 


3 


1 


Purple sweet 


WWss 

WwSs 




All 
3 








1 










White starchy 


wwSS 
wwSs 






All 
3 








1 












wwss 








All 









In the animal kingdom important work has been done in establishing 
Mendelian principles by the use of small animals, particularly mice, 
rats, guinea-pigs and rabbits. Such animals are particularly favorable 
for investigations in heredity because a large number of generations may 
be reared in a relatively short space of time. Castle has reported an 
excellent case of dihybridism in the guinea-pig. Rough coat is dominant 
to smooth and colored coat to the albino condition. When a smooth 
black is crossed wifh rough white the hybrids are rough black. In F2 



86 



GENETICS IN RELATION TO AGRICULTURE 



the segregation is in the ratio 9 rough black : 3 rough white : 3 smooth 
black : 1 smooth white. These relations are shown diagrammatically 
in Fig. 40. The factor relations are very simple. The genetic constitu- 
tion of the smooth black race is rrCC and of the rough white race RRcc, 
where R is a, factor for rough coat and its allelomorph r a factor for 
smooth coat, and C and c are factors for colored and albino coat respec- 






F. 









^F, 



Fig. 40. — Results of crossing smooth white and rough black guinea-pigs. Fi is rough 
black. Fi is in the ratio 9 rough black : 3 rough white : 3 smooth black : 1 smooth white. 
(After Baur.) 

tively. The Fi RrCc is rough black because of the dominance of these 
two characters over their allelomorphs. When Fi individuals are bred 
together the F2 segregates in accordance with normal dihybrid expecta- 
tions as shown in the checkerboard in Fig. 41. 

This experiment shows how easily new races may be established, for 
in F2 two entirely new combinations of characters were obtained, namely, 
rough black and smooth white. Of the rough black individuals only 



INDEPENDENT MENDELIAN INHERITANCE 



87 



one in nine were homozygous for both dominant factors as may be 
determined from the checkerboard analysis. Consequently for this com- 
bination of characters it would be necessary to make extensive tests of 
the individuals in order to determine their genetic constitutions. Mat- 
ing those which had been determined to be of the genetic constitution 
RRCC together would insure the production of a race of rough black 
guinea-pigs which would breed true for these characters. On the other 
hand, all those which are smooth white are of the genetic constitution 
rrcc; they are therefore homozygous and will produce a uniform progeny 
when bred together. 

Dihybridism in Drosophila. — We shall not attempt to follow out the 
chromosome relations for the guinea-pig hybrid because they are exactly 





RC 


Re 


rC 


re 


RC 


RRCC 
rough black 


RRCc 
rough black 


RrCC 
rough black 


RrCc 
rough black 


Re 


RRCc 
rough black 


RRec 
rough white 


RrCc 
rough black 


Rrcc 
rough white 


rC 


RrCC 
rough black 


RrCc 
rough black 


rrCC 
smooth black 


rrCc 
smooth black 


re 


RrCc 
rough black 


Rrcc 
rough white 


rrCe 
smooth black 


rrcc 
smooth white 



Fig. 41. — Checkerboard showing F2 segregation in the cross, smooth black X rough white 

in guinea-pigs. 



the same as those in maize. In Drosophila, however, the peculiar rela- 
tions displayed by the sex-chromosomes gives more striking instances 
of parallelism of chromosome behavior and factor distribution. The 
inheritance of white-eye color in Drosophila has already been described 
in detail. Another character, vestigial wings, shows a different type of 
inheritance. When vestigial-winged flies are crossed with normal long- 
winged flies the Fi flies of both sexes are long-winged in the reciprocal 
crosses, and in F2 segregation is in the ratio of 3 long : 1 vestigial in both 
sexes. The factor for vestigial wings, therefore, must be located in one of 
the pairs of autosomes. We shall call this factor v and its normal allelo- 
morph in the long-winged race V. The formula for a vestigial-winged 
white-eyed female then becomes vv (wX) {wX) and for a long-winged red- 
eyed male VV{WX)Y. 

The chromosome relations involved in crossing a vestigial white 
female and a long red male are shown diagrammatically in Fig. 42, Two 
pairs of chromosomes are involved, the sex-chromosomes and a pair of 



88 



GENETICS IN RELATION TO AGRICULTURE 



autosomes, A vestigial white female produces eggs all of which contain 
an X-chromosome bearing the factor w and an autosome bearing the fac- 
tor V. Half the sperms, on the other hand, contain an A'-chromosome 
bearing the factor W and an autosome bearing the factor V, and half con- 
tain a F-chromosome and an autosome bearing the factor V. Conse- 
quently when the sperm cells which contain X-chromosomes fertilize 
the eggs, Fi females of the genetic constitution Vv(WX){wX) will be 



Fi 



06 




V w 



01 61 



fly 

80 

oe 







flfflflP 



Long Red ^ 



BOO 




■Long Red ? 



0606 



Long White ? 



Long White ? 



V W 



QQ 



Long Red ? 









V w 



Vestigial 
Red $ 








w V w 



Long White ? 



360666666601 



Vestigial 
White ? 



QOQtQQRt 



Long Red d' 



001 



Long Red c? 



Long White d 



Long White cf 



Long Red d* 



Boei 



Vestigial Red cT 



08000960 



Long White d" 




V W V 







Vestigial 
White d* 



Fig. 42. — Fj gametes and F2 zygotes from the cross, vestigial white female X long red 
male. Factor symbols: v = vestigial, V = long, w = white, W = red. 

produced. Phenotypically they will be long red. When the sperm 
cells which contain F-chromosomes fertilize the same kind of eggs the 
Fi males which result will be of the genetic constitution Vv(wX)Y. 
Phenotypically they will be long white. The reduction divisions in the 
Fi female will result in the production of four kinds of eggs as 
shown diagrammatically in the figure. They will be of the genetic 
constitutions : 

V(WX) v{WX) V{wX) viwX). 

Similarly four kinds of sperm cells will be produced by the Fi male 
and they will be of the genetic constitutions: 

ViwX) viwX) VY vY. 



INDEPENDENT MENDELIAN INHERITANCE 



8^ 



The F2 population produced by mating two Fi individuals will be 
made up of Fi gametes as shown in the checkerboard analysis in Fig. 
43. When like phenotypes are collected the ratio in each sex is 3 long 
red : 3 long white : 1 vestigial red : 1 vestigial white. This is very 
different from the typical 9:3:3:1 ratio of a dihybrid, but it is strictly 
in agreement with experimental observations and chromosome behavior. 

The reciprocal cross gives different results and for that reason we 





V{wX) 


v{ivX) 


VY 


vY 


V{WX) 


VV{WX){wX) 
long red 9 


Vv{WX){wX) 
long red 9 | 


VV{WX)Y 
long red cT 


Vv{WX)Y 
long red cf 


V{wX) 


VV{wX){wX) 
long white 9 


Vv{wX){wX) 
long white 9 


VViwX)Y 
long white cT 


Vv{wX)Y 
long white cf 


v{WX) 


Vv{WX){wX) 
long red 9 


vv{WX){wX) 
vestigial red 9 


Vv{WX)Y 
long red cf 


vviWX)Y 
vestigial red cf 


v{wX) 


Vv{wX){wX) 
long white 9 


vviwX){wX) 
vestigial white 9 


Vv{wX)Y 
long white c^ 


vv{wX) Y 
vestigial white d' 



Fig. 43. — Checkerboard analysis of the F2 obtained by crossing vestigial white 9 with 

long red cf in Drosophila. 



V{WX) 



v{WX) 



VY 



vY 



VViWX){WX) 
long red 9 


Vv{WX){WX) 
long red 9 


VV{WX)Y 
long red d' 


VviWX)Y 
long red cf 


VV{WX)iwX) 
long red 9 


VviWX)iwX) 
long red 9 


VV{wX)Y 
long white cf 


Vv{wX)Y 
long white cf 


VviWX){WX) 
long red 9 


vv{WX){WX) 
vestigial red 9 


VviWX)Y 
long red cf 


vv(WX)Y 
vestigial red cf 


Vv(WX){wX) 
long red 9 


vv{WX)(ivX) 
vestigial red 9 


Vv{wX)Y 
long white cf 


vviWX)Y 
vestigial white cf 



V{WX) 



ViwX) 



viWX) 



v{wX) 



Fig. 44. — Checkerboard analysis of the F2 obtained by crossing long red 9 with vestigial 
white cf . Reciprocal of cross analyzed in Fig. 43. 

shall go through it briefly to show that this difference is a necessary con- 
sequence of the chromosome behavior. When a long red female, genetic 
formula VV(WX){WX), is mated to a vestigial white male, vv(wX)Y, the 
Fi individuals are long red females, Vv(WX)(wX), and long red males, 
Vv{WX)Y. The Fi females then give the same four types of eggs as 
those of the reciprocal cross, viz., 

ViWX) V{wX) viWX) viwX). 



90 



GENETICS IN RELATION TO AGRICULTURE 



The males, however; produce the following series of sperms: 

V{WX) v{WX) VY vY. 

Mating Fi flies of this cross, therefore, results in the F^ population shown 
in the checkerboard in Fig. 44. When these are collected into like pheno- 
types the ratio obtained is 9 long red: 3 long white: 3 vestigial red: 

1 vestigial white, but this agreement with the typical dihybrid ratio is only 
apparent. When the females alone are considered the ratio is 6 long red: 

2 vestigial red, and the males are in the ratio 3 long red: 3 long white: 
1 vestigial red : 1 vestigial white. The disturbance in the ratio is 
occasioned by the unique behavior of the white-eye character which 
behaves exactly as it did in the simple case which was analyzed previously. 
The reciprocal ratio, therefore, is additional evidence as to the adequacy 
of the chromosome theory. 

The Trihybrid.- — The same line of reasoning of course applies to 
cases in which three pairs of factors are involved. Such for example is a 
case which Baur has described in the common garden snapdragon. 
Antirrhinum majus. In this particular case the factors involved have 
the following relations. 

Z — a factor which conditions the development of the zygomorphic 
type of blossom which is characteristic of the species. The factor z, its 
allelomorph, conditions the production of peloric blossoms, i.e., blossoms 
which display radial symmetry. The normal form is nearly completely 
dominant. 

R — a factor for red color of the blossoms. The allelomorph r gives 
flowers which are flesh colored. 

I — a factor for ivory coloration of the blossoms. The allelomorph 
i in this case conditions the production of yellow flowers. R with I gives 
flowers red on an ivory background, a magenta type of coloration, 
whereas R with i gives flowers which are red on a yellowish background. 
It is possible to distinguish these two classes in a mixed population. 

Table XIII. — ^Distribution of Classes Among the Progeny of an Fi H^'brid 
Snapdragon of the Composition ZzRrIi 



Ratio 



Phenotypes 



Observed 



Expected 



27 
9 
9 
3 
9 
3 
3 
1 



Zygomorphic, red on ivory 

Zygomorphic, red on yellow 

Zj'gomorphic, flesh-colored, on ivory. . 
Zygomorphic, flesh-colored, on yellow 

Peloric, red, on ivory 

Peloric, red, on yellow 

Peloric, flesh-colored, on ivory 

Peloric, flesh-colored, on yellow 



64 

14 

10 

23 

6 

1 

4 

2 



52 

17 

17 

17 

6 

6 

6 

2 



INDEPENDENT MENDELIAN INHERITANCE 



91 



The 7^1 hydrid ZzRrIi was the noriiml flower form and produced 
blossoms of a magenta coloration, i.e., red on ivory. From self-fertilized 
seed of such plants there were produced 124 plants distributed with 
respect to their phenotypes as shown in Table XIII. For so small a popu- 
lation, in number of individuals only about twice that necessary to obtain 
a triple recessive, the agreement is good. 

To account for these hybrid results on a chromosome basis it is 
necessary to assume merely that the three pairs of allelomorphs are borne 
in different pairs of chromosomes. When germ cells are formed in theFi 




Fig. 45. — Diagrammatic representation of reduction divisions in an Fi snapdragon of the 

genetic constitution ZzRrli. 



liybrids the members of the three pairs of chromosomes separate independ- 
ently so that eight different kinds of gametes are formed. In Fig. 45 are 
shown diagrammatically the reduction divisions in an Fi snapdragon of 
the genetic constitution ZzRrli. Since any one of these types of re- 
duction has a good as chance of occurrence as any other one, the eight 
kinds of gametes are produced in substantially equal numbers. When 
such a plant is self-fertilized there are 64 possible combinations as shown 
in the checkerboard in Fig. 46. In the description of phenotypes in this 
checkerboard certain differences between homozygous and heterozygous 
forms have been disregarded. This is true particularly for the Rr 
individuals as contrasted with the homozygous RR individuals. The 



92 



GENETICS IN RELATION TO AGRICULTURE 



ZRI 


ZRi 


Zrl 


Zri zRI 


zRi 


zrl 


zri 


ZRI 


ZRI 
ZRI 

zygo- 

morphic 

red 

ivory 


ZRi 
ZRI 

zygo- 

morphic 

red 

ivory 


Zrl 
ZRI 

zygo- 

morphic 

red 

ivory 


Zri 
ZRI 

zygo- 

morphic 

red 

ivory 


zRI 
ZRI 

zygo- 

morphic 

red 

ivory 


zRi 
ZRI 
zygo- 
morphic 
red 
ivory 


zrl 
ZRI 

zygo- 

morphic 

red 

ivory 


zri 
ZRT 
zygo- 
morphic 
red 
ivory 


ZRi 


ZRI 
ZRi 

zygo- 

morphic 

red 

ivory 


ZRi 

ZRi 

zygo- 

morphic 

red 
yellow 


Zrl 
ZRi 

zygo- 

morphic 

red 

ivory 


Zri 

ZRi 

zygo- 

morphic 

red 
yellow 


zRI 
ZRi 

zygo- 

morphic 

red 

ivory 


zRi 

ZRi 

zygo- 
morphic 

red 
yellow 


zrl 
ZRi 
zygo- 
morphic 
red 
ivory 


zri 
ZRi 
zygo- 
morphic 

red 
yellow 


Zrl 


ZRI 
Zrl 

zygo- 

morphic 

red 

ivory 


ZRi 
Zrl 

zygo- 

morphic 

red 

ivory 


Zrl 
Zrl 

zygo- 
morphic 

flesh- 
colored 

ivory 


Zri 
Zrl 
zygo- 
morphic 
flesh- 
colored 
ivory 


zRI 
Zrl 

zygo- 

morphic 

red 

ivory 


zRi 
Zrl 

zygo- 

niorphic 

red 

ivory 


zrl 
Zrl 
zygo- 
morphic 
flesh- 
colored 
ivory 


zri 
Zrl 
zygo- 
morphic 
flesh- 
colored 
ivory 


Zri 


ZRI 

Zri 

zygo- 

morphic 

red 

ivory 


ZRi 

Zri 

zygo- 

morphic 

red 
yellow 


Zrl 

Zri 

zygo- 
morphic 

flesh- 
colored 

ivory 


Zri 

Zri 

zygo- 
morphic 

flesh- 
colored 
yellow 


zRI 

Zri 

zygo- 

morphic 

red 

ivory 


zRi 

Zri 

zygo- 

morphic 

red 
yellow 


zrl 

Zri 

zygo- 
morphic 

flesh- 
colored 

ivory 


zri 

Zri 

zygo- 
morphic 

flesh- 
colored 
yellow 


zRI 


ZRI 
zRI 

zygo- 

morphic 

red 

ivory 


ZRi 
zRI 

zygo- 

morphic 

red 

ivory 


Zrl 
zRI 

zygo- 

morphic 

red 

ivory 


Zri 

zRI 

zygo- 

morphic 

red 
ivory 


zRI 
zRI 

peloric 

red 

ivory 


zRi 

zRI 
peloric 

red 
ivory 


zrl 

zRI 

peloric 

red 

ivory 


zri 

zRI 
peloric 

red 
ivory 


zRi 


ZRI 

zRi 

zygo- . 

morphic 

red 

ivory 


ZRi 

zRi 

zygo- 

morphic 

red 
yellow 


Zrl 
zRi 

zygo- 

morphic 

red 

ivory 


Zri 

zRi 

zygo- 

morphic 

red 
yellow 


zRI 

zRi 
peloric 

red 
ivory 


zRi 

zRi 
peloric 

red 
yellow 


zrl 

zRi 

peloric 

red 

ivory 


zri 

zRi 
peloric 

red 
yellow 


zrl 


ZRI 

zrl 

zygo- 

morphic 

red 

ivory 


ZRi 
zrl 
zygo- 
morphic 
red 
ivory 


Zrl 

zrl 

zygo- 
morphic 

flesh- 
colored 

ivory 


Zri 
zrl 
zygo- 
morphic 
• flesh- 
colored 
ivory 


zRI 

zrl 
peloric 

red 
ivory 


zRi 

zrl 
peloric 

red 
ivory 


zrl 

zrl 
peloric 

flesh- 
colored 
ivory 


zri 

zrl 
peloric 

flesh- 
colored 

ivory 


zri 


ZRI 

zri 

zygo- 

morphic 

red 

ivory 


ZRi 

zri 

zygo- 
morphic 

red 
yellow 


Zrl 

zri 

zygo- 
morphic 

flesh- 
colored 

ivory 


Zri 

zri 

zygo- 
morphic 

flesh- 
colored 
yellow 


zRI 

zri 
peloric 
red 
ivory 


zRi 

zri 
peloric 

red 
yellow 


zrl 

zri 
peloric 

flesh- 
colored 

ivory 


zri 

zri 
peloric 

flesh- 
colored 
yellow 



Fig. 46. — Checkerboard Analysis of the F2 Obtained from a Cross Involving 
THE Three Pairs of Factors, Z-z, R-r, I-i 



INDEPENDENT MENDELIAN INHERITANCE 93 

former are intermediate in coloration between the full-colored RR 
individuals and the flesh-colored rr individuals, and form a distinct class 
in themselves. But disregarding these differences the phenotypes in F2 
are in the following ratio : 

27 Plants with zygomorphic, red on ivory flowers. 
9 Plants with zygomorphic, red on yellow flowers. 
9 Plants with zygomorphic, flesh-colored on ivory flowers. 
9 Plants with peloric, red on ivory flowers. 
3 Plants with zygomorphic, flesh-colored on yellow flowers. 
3 Plants with peloric, red on yellow flowers. 
3 Plants with peloric, flesh-colored on ivory flowers. 
1 Plant with peloric, flesh-colored on yellow flowers. 

This 27:9:9:9:3:3:3:1 ratio is typical for trihybrids, if dominance 
occurs in the three pairs of factors involved. Like the dihybrid ratio 
it is derivable from the monohybrid 3 : 1 ratio by subdividing the mem- 
bers of each term in the 3 : 1 ratio and then by again subdividing each 
term of the dihybrid ratio thus obtained in the ratio 3:1. To illustrate 
with our example, if the contrasted characters zygomorphic and peloric 
are considered segregation is in the ratio 3 zygomorphic: 1 peloric. 
When the contrasted characters red against flesh-colored, which also 
segregate in the simple ratio, are introduced into the analysis the ratio 
becomes 3 zygomorphic (3 red : 1 flesh-colored) : 1 peloric (3 red : 1 
flesh-colored) = 9 zygomorphic red : 3 zygomorphic flesh-colored : 3 
peloric red : 1 peloric flesh-colored. When finally the contrasted 
characters ivory against yellow are introduced this becomes 9 zygomorphic 
red (3 ivory : 1 yellow) : 3 zygomorphic flesh-colored (3 ivory : 1 yellow) : 
3 peloric red (3 ivory : 1 yellow) : 1 peloric flesh-colored (3 ivory : 1 yellow) . 
This gives the final distribution tabulated above. 

It is important to note that in this phenotypic ratio one member only 
of each phenotype is homozygous for all its factors and will breed true 
thereafter. From a Mendelian standpoint an individual is either homo- 
zygous or heterozygous for a given factor, if it is homozygous it is pure 
bred with respect to that factor and will breed true thereafter, irrespec- 
tive of its derivation. It is therefore possible in F2 to obtain a pure race 
with respect to any combination of parental factors provided only that 
a large enough F2 generation is grown and tested. The increasing diffi- 
culty of fulfilling these conditions as the number of factors involved 
increases is obvious, so that from the standpoint of practicability it is 
usually necessary to work with crosses involving a relatively small num- 
ber of factor differences. 

Another fact which is apparent from this trihybrid case is the 
greater ease with which homozygous individuals may be obtained from 
the classes which are represented in the smallest numbers. In the above 



94 



GENETICS IN RELATION TO AGRICULTURE 



example peloric snapdragons with flowers flesh-colored on yellow are 
least frequent, but they all breed true on self-fertilization. In the case 
of the most frequent class, however, the zygomorphic red on ivory only 
one plant in 27 is homozygous for the three factors involved and con- 
sequently would breed true. These are features of Mendelism which 
have a direct practical application. 

Multi-factor Hybrids. — Very few cases have been worked out which 
demonstrate conclusively, that more than three pairs of independently 
Mendelizing characters were involved. Not only are the experimental 
difficulties in such cases too great, but the scientific interest attached to 
them is not considerable. From a scientific standpoint accuracy of analy- 
sis is of chief importance, and accuracy is best attained by working with 
small numbers of factors at a time. 

Little and Phillips, however, have conducted an experiment involving 
four pairs of independently Mendelizing factors in mice. The factors 
and the character expressions which they produce are listed below: 

A — factor for agouti coloration. In this type of coloration the pigment is disposed 
in bands in the hairs giving the peculiar gray or agouti coloration of the wild mouse. 
The allelomorph a conditions a uniform distribution of pigment in the hairs. 

B — a factor for black coat color. In this experiment the allelomorph b conditions 
the production of brown coat color. 

D — -a factor for intensity of coat coloration. Animals with the factor D were full 
colored, whereas those with d were "dilute" colored. 

P — a factor for eye coloration. P conditions a dark eye coloration; the allelo- 
morph p pink-eye color. 



Table XIV. — Four-fold Factor Segregation of Mice {From Little and Phillips) 



Phenotype 



Formula 


Observed 


Expected 


ABDP 


436 


373 


aBDP 


127 


124 


AbDP 


103 


124 


ABdP 


130 


124 


ABDp 


103 


124 


abDP 


40 


41 


AbdP 


31 


41 


aBdP 


37 


41 


aBDp 


35 


41 


AbDp 


38 


41 


ABdp 


38 


41 


abdP 


11 


14 


abDP 


12 


14 


Abdp 


15 


14 


aBdp 


17 


14 


abdp 


7 


5 



Observed 
ratio 



Theoretical 
ratio 



Black agouti 

Black 

Brown agouti 

Dilute black agouti 

Pink-eyed black agouti 

Brown 

Dilute brown agouti 

Dilute black 

Pink-eyed black 

Pink-eyed brown agouti 

Pink-eyed dilute black agouti . 

Dilute brown 

Pink-eyed brown 

Pink-eyed dilute brown agouti 

Pink-eyed dilute black 

Pink-eyed dilute brown 



94.5 

27.5 

22.3 

28.2 

22.3 

8.7 

6.7 

8.0 

7.6 

8.2 

8.2 

2.4 

2.6 

3.3 

3.7 

1.5 



81 

27 

27 

27 

27 

9 

9 

9 

9 

9 

9 

3 

3 

3 

3 

1 



INDEPENDENT MENDELIAN INHERITANCE 95 

For the experiment a wild male of the genetic formula AABBDDPP 
was mated to a pink-eyed dilute brown female of the genetic constitution 
aabbddpp. The Fis, AaBbDdPp, displayed all four dominant characters 
and were like the wild males. The F2 segregation is shown in Table XIV. 
For 1180 individuals only about four times the number of genetic com- 
binations for a four-factor hybrid, the agreement is satisfactory. 

As for the chromosome interpretation, it may be made in the same 
way as in other cases by assuming that four different pairs of chromo- 
somes bear the factors. Sixteen different kinds of gametes would be 
formed by such a hybrid, and these together would give the 256 gametic 
combinations of the F2 generation. 

For higher numbers of pairs of factors the same manner of independ- 
ent distribution may hold as for those cases which have been outlined in 
detail. For independent distribution, the chromosome condition is 
simply that the different pairs of factors be borne in different pairs of 
chromosomes. Since, however, the total number of factors in any species 
must greatly exceed the number of pairs of chromosomes, it cannot be ex- 
pected that every multi-factor hybrid will display independent segregation 
for all its factors. The number of pairs of chromosomes in Drosophila is 
four, consequently no crosses in this species involving more than four pairs 
of factors can possibly display independent segregation, if the chromosome 
theory be valid. Moreover, on the basis of the laws of probability, the 
chances that any particular case of even fourfold factor hybridization 
in this species would display independent segregation are extremely 
slight. Abundant evidence in this species has established the validity 
of these theoretical deductions. The same principles may logically 
be extended to other species so that for even as small a number of pairs 
of factors as five in wheat, which has eight pairs of chromosomes, in peas 
which have seven, and in corn which has ten, independent segregation 
would be an exception rather than the rule. Cases where independent 
segregation does not occur are treated in the next chapter, which deals 
with linkage. 

Methods of Dealing with Genetic Data. — Many different methods 
have been devised for representing the results of Mendelian studies, and as 
yet the work of any large group of investigators is marked by a consider- 
able lack of uniformity in this respect. Often the same investigator 
employs at one time one method of representation, and at another time, 
another. This is as it should be, for it can hardly be expected in a field 
of investigations marked by as rapid strides as had been characteristic of 
genetics in recent years that the ideal method of presentation should have 
been discovered while only a comparatively small portion of the evidence 
is at hand. Moreover, the method of presentation is merely a short- 
hand account of the operation of certain principles; it should not, there- 



96 



GENETICS IN RELATION TO AGRICULTURE 



fore, greatly matter what methods are adopted so long as they represent 
clearly and adequately the operation of these principles. It is necessary 
for the student to familiarize himself with at least some of the more 
widely employed methods, remembering that with the basic principles 
clearly in mind, it should be possible to interpret very easily the methods 
employed in the presentation of any body of Mendelian data. 

For ordinary work it is well to have a definite system of interpreting 
problems. In the following treatment only cases involving the category of 
independently segregating pairs of factors are considered, but it should be 

possible to extend the system without great 
difficulty to other categories as will be pointed 
out in those sections dealing with such 
categories. 

When an individual is heterozygous for 
one pair of factors two types of gametes are 
possible. If the factors involved are repre- 
sented by A and a, the genotypic constitution 
of such an individual is Aa, and the two 
types of gametes are A and a. If an in- 
dividual is heterozygous for two paifs of 
factors, its genotypic formula will be AaBb, 
and there are then possible four different kinds 
of gametes, namely AB, Ab, aB and ab. As 
the number of heterozygous factors increases, 
therefore, the number of possible combinations 
of factors increases geometrically so that it is 
necessary to adopt a method of writing 
down these possible combinations. The 
method of dichotomy may be used in such 
cases, and the diagram, Fig. 47, explains its operation without further 
comment. 

With as few as five pairs of factors, therefore, there are possible no 
ess than thirty-two different kinds of gametes as follows: 




Fig. 47. — Method of writing 
all possible combinations of 
any number of pairs of factors. 



ABODE 


AbCDE 


aBCDE 


abCDE 


ABCDe 


AbCDe 


aBCDe 


abCDe 


ABCdE 


AbCdE 


aBCdE 


abCdE 


ABCde 


AbCde 


aBCde 


abCde 


ABcDE 


AbcDE 


aBcDE 


abcDE 


ABcDe 


AbcDe 


aBcDe 


abcDe 


ABcdE 


AbcdE 


aBcdE 


abcdE 


ABcde 


Abode 


aBcde 


abode 



By following this method consistently it is possible to write out readily 
the possible combinations of any series of factors. Fortunately or un- 



INDEPENDENT MENDELI AN INHERITANCE 97 

fortunately the limits of experimental facilities usually preclude the pos- 
sibility of working with large numbers of factors in any single experi- 
ment, so that it rarely becomes necessary to handle any large number of 
combinations. 

Since Fz results are commonly obtained by selfing the Fi individuals 
in the case of plants, or interbreeding them in the case of animals, the 
F2 ratios ordinarily represent the product of two like gametic series each 
consisting of all possible combinations of the different factors involved. 
There are several methods of obtaining these ratios, each of which has 
its special advantages. The simplest of these is the algebraic method 
which merely depends upon the multiplication of the two series together 
as illustrated in the following general example for two factor differences. 

Female gametes AB -^ Ab -{- aB + ah 
Male gametes AB -\- Ah -\- aB -\- ah 

F-i zygotes: 

AABB + AABb + AaBB + AaBb 

AABb + AaBh +AAhh + Aahb 

AaBB + AaBb + aaBB + aaBh 

AaBb + Aabb + aaBb + aabh 

F2 genotypes: 
AABB + 2 AABb + 2AaBB + AAaBb + A Abb + 2 Aabb + aaBB + 2aaBb + aabb 

Collecting these F2 genotypes into their respective phenotypes we get 
the following results: 



9A5 


3Ab 


ZaB 


lab 


lAABB 


lAAbb 


laaBB 


laabb 


2AABb 


2Aabb 


2aaBb 




2AaBB 








^AaBb 









This tabulation of the genotypes since it shows that the genotypes 
within a phenotype are in definite ratios to each other immediately sug- 
gests the method of progression of writing down the F2 phenotypic and 
genotypic distributions on the basis of the symmetrical relations displayed 
by them. The ratio of phenotypes in F2 in a cross involving n pairs of 
factors is conveniently obtained in cases of complete dominance by the 
expansion of the expression (3 + 1)" or by continuously dividing the 
terms of a simpler ratio in the ratio 3 : 1 until the number of pairs of factor 
differences involved is satisfied. In the following table the phenotypic 
ratios obtained by the expansion of (3 -|- 1)" for values of n up to five 
have been given in condensed form. 
7 



98 GENETICS IN RELATION TO AGRICULTURE 

Table XV. — Phenotypic Ratios Obtained by Expansion op the Binomial 

(3j+ D". 



Pairs of 
factors 


(3 + D" 


Phenotypic ratio 


Number of 
combinations 


1 
2 
3 
4 
5 


(3 + 1)1 
(3 + 1)2 
(3 + 1)3 
(3 + D* 
(3 + 1)5 


3 +1 

32 + 2.3 + 1 

33 + 3.32 + 3.3 + 1 

3* + 4.33 + 6.32 + 4.3 + 1 

35 + 5.3* + 10.3^ + 10.32 + 53^1 


4 

16 

64 

256 

1,024 




(3 + 1)» 


nn , ,^nn , , "('^ ' D „ n - 2 , n(n - 1) (n - 2) 


4" 




J -t ll.J +18 3 1 13 +..+1 



For three pairs of factors, therefore, we interpret this table to mean 
that the distribution with respect to phenotypes is as follows : 

27ABC:9ABc:9AhC:9aBC:Mhc:3aBc.3ahC:lahc 

If it is desired now to write down the numbers of each particular geno- 
type in a given phenotype, the procedure according to the method of pro- 
gression is very simple. Let us select the class 27 ABC the genotypes of 
which are as follows: 



lAABBCC 
2AABBCC 
2AABbCC 
2AaBBCC 



AAABhCc 
4:AaBBCc 
4:AaBbCC 
SAaBbCc 



It may be noted that there is one phenotype in each class homozygous 
for all its factors. In this class starting with this phenotype, we double 
the number of individuals each time an additional pair of factors becomes 
heterozygous. Thus there are three genotypes possible with only one 
heterozygous factor, and there will be two individuals of each of these, 
there will be three different genotypes having two heterozygous factors, 
and each of these will be represented by four individuals, and finally 
there is only one genotype -with three heterozygous factors and it is 
represented by eight individuals. The method of progression is based 
upon the symmetrical relations which exist in the phenotypic ratios and 
in the ratios of genotypes within a phenotype and is a very convenient 
method for general use. 

For illustrative purposes when it is desired to bring out relations 
graphically the checkerboard method of Punnett is much used. This 
method has already been employed in this book and needs no extended 
discussion here. The accompanying general checkerboard for three 
pairs of factors will illustrate the relations obtaining when this method is 
employed consistently. As shown in Fig. 48 the gametic series is written 



INDEPENDENT MEN DELI AN INHERITANCE 



99 



down at one side and at the top of the checkerboard and any square is 
filled out by writing down the genetic formula of the gamete at the top 
of its column and the one at the end of its row. If the series are written 
in the order shown the diagonal 1-3 will pass through all homozygous 
combinations. The number of these is evidently equal to the number 
of possible combinations in the gametic series. The diagonal 2-4 
passes through all those combinations in which all three pairs of factors 

1 ABC ABc AhC Ahc ' aDC aDc abC abc 2 



ABC 



ABc 



AbC 



Abc 



aBC 



aBc 



abC 



abc 



\bc 


ABc 
ABC 

X 


AbC 
ABC 

X 


Ahc 
ABC 

X 


aBC 
ABC 

X 


aBc 
ABC 

X 


abC 

ABC 

X 


abc/ 
ABC 

A 


ADC 
ABc 

X 


\bc 
2>Bc 


AbC 
ABc 

X 


Abc 
ABc 


aBC 

ABc 

X 


aBc 
ABc 


abC 
ABc 


abc 
ABc 


ABC 

AbC 

X 


ABc 
AbC 

X 


\AbC 
IbC 


Ahc 
AhC 


aBC 
AbC 

X 


aB</ 

aIc 


ahC 
AbC 


abc 
AhC 


ABC 

Abc 

X 


ABc 

Ahc 


AbC 
Abc 


XAbc 
Abc 


aBC 

a/c 

A 


aBc 

Ahc 


ahC 
Abc 


abc 
Ahc 

( 


ABC 
aBC 

X 


ABc 
aBC 

X 


AbC 
aBC 

X 


Ahc/ 
aBC 


\bc 

aBC 


aBc 
aBC 


abC 
aBC 


ahc 
aBC 


ABC 
aBc 

X 


ABc 
aBc 


AbC^ 

/ 
aBc 


Abe 
aBc 


aBC 
aBc 


\bc 
oBc 


abC 

aBc 


abc 
aBc 


ABC 
abC 

X 


ABc! 
abC 


AbC 
abC 


Abc 
abC 


aBC 
abC 


aBc 

abC 


\bC 
abC 


abc 

abC 


ABC^ 
ab'c 


ABc 
abc 


AbC 
ahc 


Abc 
abc 


aBC 

abc 


aBc 
abc 


abC 
abc 


\ibc 
abc 



Fig. 48. — Checkerboard method of analyzing expected results in F2 from a cross involving 
three pairs of allelomorphs. The " x " zygotes belong to phenotype ABC. Cf. p. 98. 

are heterozygous, and the number of these is also equal to the number of 
kinds of gametes. The bottom row gives the ratio which would be ob- 
tained by crossing the Fi back to the triple recessive form. The student 
will be able to determine other relations existing in checkerboards of this 
type. 

From a mathematical standpoint, students of genetics are interested 
in two things, the number and proportion of various types of individuals, 



100 



GENETICS IN RELATION TO AGRICULTURE 



and in methods of testing the mathematical vaHdity of segregation ratios. 
Table XVI gives the mathematical relations which obtain in the pro- 
duction of gametes in i^i individuals and in their union to form the F2 
zygotes. It is assumed throughout that one factor of each pair of 
allelomorphs is dominant. 



Table XVI. — Proportions Existing 
Various Numbers op 


[N Mendelian Experiments Inv^olving 
Factor Differences 


Number of pairs of factors 


1 2 


3 


4 


5 


6 


n 


Number of different kinds of gametes. . . . 

Number of combinations of gametes 

Number of homozygotes in F^ 


2 
4 
2 
2 
3 
2 

1 


4 
16 

4 
12 

9 

4 

5 


8 
64 

8 
56 

27 
8 

19 


16 
256 

16 
240 

81 

16 

65 


32 

1,024 

32 

992 

243 

32 

211 


64 
4,096 

64 

4,032 

729 

64 

665 


2» 
4» 

2" 


Number of heterozygotes in F-> 


4" - 2" 


Number of kinds of genotypes in F2 

Number of kinds of homozygous genotypes 
Number of kinds of heterozygous geno- 
types 


3" 

2" 

3" - 2" 



From this table it is clearly apparent how rapidly Mendelian problems 
increase in complexity with increases in the number of factor differences. 
With only five pairs of factors the number of individuals necessary to 
represent the F2 population is 1024 and in order to be sure to have all 
classes represented it would be necessaty to grow four or five times as 
many individuals as this. In such an experiment there would be 243 
different genotypes distributed among thirty-two phenotypes. Natu- 
rally the chances of selecting a homozygous individual would vary ac- 
cording to the phenotype within which such selection was made, but the 
average chance of selecting a homozygote would be one in thirty-two, 
and the chance of selecting such an individual in the class displaying all 
five dominant characters would be only one in 243. The practical diffi- 
culties of dealing with large numbers of factor differences are there- 
fore of considerable importance in planning and carrying out Mendehan 
experiments. 

Methods of testing the "goodness of fit" of Mendelian ratios depend 
upon the application of the mathematical theory of probabilities. It 
is beyond the province of this book to enter into any exhaustive treat- 
ment of this subject, the present discussion is intended merely to point 
out the mathematical requirements which must be fulfilled, if no factors 
are present which tend to disturb the ratio constantly in a given direc- 
tion. For most problems of this kind it is sufficiently accurate to con- 
sider the standard deviation of a Mendelian ratio = ±y/N{K —N 
where N represents a particular term of a Mendelian ratio and i^repre- 



INDEPENDENT MENDELIAN INHERITANCE 



101 



sents the sum of all the terms of such a ratio. This gives for the probable 
error E,, of a given term .V of a Mendelian ratio the value 



En= ± 0.6745 



V 



N{K -N) 



n 



In this formula n = the total number of individuals classified. 

The actual application of this formula may be illustrated by the use 
of data from East and Hayes given in Table XI. The totals in this table 
give observed frequencies as shown in Table XVII. 

Table XVII. — Goodness of Fit in a Mendelian Experiment 



Phenotypes 


Observed 


Observed 
ratio 


Theoretical 
ratio 


E 


Probability 


Purple starchy .... 

Purple sweet 

White starchy 

White sweet 


1,861 
614 
548 
217 


9.190 
3.032 
2.706 
1.072 


9 
3 
3 
1 


±0.104 
±0.074 
±0.074 
±0.046 


1:3.45 
1:1 

1:142.26 
1:2.57 


Totals 


3,240 


16.000 


16 











The results are expected to be in agreement with a 9:3:3:1 ratio; 
therefore these observed results are first reduced to the form of a ratio 
per 16 by dividing each term by j^e of the total number of individuals, 

3240 
or by "Yr' ~ 202.5. By this method the observed ratio in Table 

XVII was calculated. 

To obtain the probable error for the purple starchy class values are 
substituted in the above formula as follows : 



£"9 = ± 0.6745 



V 



9(16 - 9) _ 



3240 



= ± 0.104 



The observed deviation 0.19 is approximately twice the value of the 
probable error. For practical purposes a deviation less than three or four 
times the probable error is not considered significant. A deviation of the 
above magnitude in comparison to the probable error occurs about once 
in four times. In Table XVII the values of the probable error have been 
calculated for all four of the terms of this ratio. One term lies considerably 
within the probable error and its probability has been put down as 1:1. 
This is not strictly correct but serves the purposes of these calculations. 
It will be noted that there is one serious deviation, that of the white 
starchy class which could occur only once in 142 times. This deviation 
is not serious enough, however, to lead us to reject the hypothesis of 
two factor differences for this case, but it may indicate that other dis- 
turbing forces are in operation in this experiment. 



102 



GENETICS IN RELATION TO AGRICULTURE 



A better method of testing goodness of fit has been suggested by 
Harris. The formula employed is 



X2= 2 



(o - c)' 



In this formula o = the observed frequency of any class; c, the cal- 
culated frequency of that class; and 2 indicates that all values of the 

type ^^ — are added together. When this formula is applied to the 

case treated above the values obtained are as given in Table XVIII. The 
value of X^ is 8.14. The number of phenotypic classes is four. To deter- 
mine the significance of this value it is necessary to refer to Elderton's 
tables for calculating goodness of fit. The value for P, the probability, 
for this case derived from such a table is 0.0437. The chances that the 
deviations shown in this ratio are merely due to random sampling are 
about one in twenty-three, again confirming our previous statement 
that some unknown slightly disturbing forces may be operating in this 
case. The deviation, however, is not enough to establish this certainly, 
for such a deviation might be expected to occur in about 4 per cent, of 
cases. 



Table XVIII. — Goodness op Fit in a Mendelian Experiment 



Phenotypes 



Observed 



Calculated 



(o - c)2 



Purple starchy 
Purple sweet. . 
White starchy. 
White sweet . . . 



o 
1,861 
614 
548 
217 



3,240 



c 

1,822.5 

607.5 

607.5 

202.5 



3,240.0 



0.81 
0.07 
5.83 
1.43 



14 =X2 



Mathematically the method suggested by Harris is preferable. It 
has also the advantage that it gives a measure of the goodness of fit of the 
ratio as a whole; which particular terms are most seriously at variance 

((J _ C)2 

may be determined by simple inspection of the values of . For 

determining the significance of X^, it is necessary to have available 
Elderton's table for test of goodness of fit. These are given in Pearson's 
tables for statisticians and biometricians. It must ever be held in 
mind that forces which tend to disturb Mendelian ratios may not neces- 
sarily be of significance as bearing upon the essential feature of the analy- 
sis, namely, that a given number of independent factors are concerned 



INDEPENDENT MENDELIAN INHERITANCE 103 

in a certain experiment. There is always a chance that biological 
conditions of necessity may disturb a ratio, for after all a ratio is only 
the end point of a series of phenomena which we pretend to describe 
step by step. Unless constantly guarded against, such biological con- 
ditions as differences in viability, variations in phenotypic expression, 
etc., may result in selective elimination of a certain number of zygotes 
at some time previous to classification, or in error in the classification 
of some individuals. 



CHAPTER VI 

LINKAGE RELATIONS IN MENDELISM 

Thus far Mendelian experiments have been considered in which the 
different pairs of factors segregate independently, and it has been shown 
that such cases may be explained very simply on the assumption that 
different pairs of chromosomes carry independent factors. However, 
there are several different species of plants and animals in which the 
number of known factor differences exceeds the number of pairs of 
chromosomes. Since it is reasonable to believe that only a small 
proportion of the possible number of factorial differences in any species 
has been analyzed, the conclusion appears justifiable that the number 
of factors in any species of plant or animal greatly exceeds the number of 
pairs of chromosomes; in fact our present evidence leads us to believe 
that the number of hereditary units in any organism must reach into the 
thousands. If the chromosome view of heredity is valid, therefore, 
each chromosome must carry a very great number of factors. In the 
present chapter it is proposed to discuss that class of Mendelian phe- 
nomena which depend upon factors which tend to remain together during 
segregation rather than to undergo independent assortment. Assuming 
that such factors are borne by the same chromosome, it will be shown 
how the chromosome mechanism provides an adequate physical basis 
for all the relations exhibited by such factors. Linkage and factor 
coupling are terms applied to that type of inheritance in which the 
factors tend to remain together in segregation. Linkage of factors is 
definitely an exception to one of the principles which Mendel laid down, 
namely, that of independent character segregation. Nevertheless by 
common consent the term Mendelism has been extended to include all 
phenomena of inheritance based on the unit factor hypothesis. For a 
long time only a few cases of linkage were known, and these were regarded 
in effect as anomalies. But the advocates of the chromosome theory of 
heredity have zealously prosecuted the study of linkage because of the 
many ways in which linkage relations parallel chromosome behavior. 
Moreover as the number of definitely recognizable factors within a 
species increases it becomes more and more important to determine the 
relations which the factors display among themselves. Linkage relations 
among factors, therefore, are of primary importance, and have been the 
direct means of giving us a clear and illuminating picture of the consti- 

104 



LINKAGE RELATIONS IN MEN DELI SM 



105 



tution of the hereditary material and of the operation of the chromosome 
mechanism in the distribution of the herecHtar}^ units. 

Purple Aleurone and Waxy Endosperm in Maize. — A typical example 
of the relations which obtain for linked factors is given in the experiments 
which involve purple aleurone color and waxy endosperm in maize. We 
have shown in Chapter V that aleurone color in maize in certain cases 
depends on a single factor difference, so that in F2 segregation is in the 
ratio 3 purple : 1 white. For waxy endosperm, when contrasted with 
starchy endosperm, Collins has shown that starchiness is dominant and 
that in Fz segregation is in accordance with the normal monohybrid 
ratio, 3 starchy : 1 waxy. The factors involved in these two cases are 
C for aleurone coloration and its recessive allelomorph c for colorless 
aleurone, and W for starchy endosperm and its recessive allelomorph 
w for waxy endosperm. Collins found that when purple starchy corn, 
CCWW, is crossed with white waxy, ccww, Fi is purple starchy, CcWw; 
but jP2 does not segregate in the expected dihybrid ratio 9 purple starchy: 
3 purple waxy : 3 white starchy : 1 white waxy. The data which he 
actually obtained from six ears are given in Table XIX. The calculated 

Table XIX. — F2 Segregation of Cross Purple Starchy X White Waxy 

(After Collins) 



Ear number 


Number of 


Purple 


Purple 


White 


White 


grains 


starchy 


waxy 


starchy 


waxy 


152 


183 


112 


20 


22 


29 


301 


579 


372 


62 


63 


82 


302 


536 


343 


52 


53 


88 


303 


627 


409 


57 


62 


99 


325 


650 


434 


55 


61 


100 


380 


161 


104 


17 


18 


22 


Observed totals . 


2,736 


1,774 


263 


279 


420 


Calculated 


.9:3:3:1 


1,539 


513 


513 


171 


Calculated 22.6 ( 


?rossing-over . . . 


1,775 


276 


276 


409 



ratio based on independent segregation evidently falls far short of agree- 
ment with the observed results, even though, when each pair of characters 
is considered separately, the agreement with the monohybrid ratio is 
very satisfactory. Thus for purple and white the observed totals are 
2037:699 giving a ratio of 2.98:1.02, and for starchy and waxy the 
observed totals are 2053:683 giving a ratio of 3.00:1.00. The latter 
ratio is so close that it would be perfect if only one kernel were shifted 
from the starchy to the waxy class. Taking each pair separately, there- 



106 GENETICS IN RELATION TO AGRICULTURE 

fore, the factors evidently segregate in the normal Mendelian fashion, 
but the excess of purple starchy and white waxy kernels indicates that 
the factors C and W which came from one parent and c and w which came 
from the other have been distributed to the same gametes more often 
than would occur on the basis of independent segregation. 

The ordinary gametic ratio for independent segregation in a hybrid 
of the genetic constitution CcWw is 

lCW:lCw:lcW:lcw. 

In this particular case, however, the gametes were produced in about 
the ratio 

3ACW:lCw:lcW:3Acw. 

The factors, therefore, display partial linkage, i.e., the parental com- 
binations of factors tend to remain together more frequently than they 
tend to form new combinations. The factor W breaks away from C to 
form a new combination with c only once in about 4.4 times, instead of 
once in two times as is the case for independent segregation. Neces- 
sarily whenever W breaks away from C to form a new combination with 
c, w forms a new combination with C. This accounts for the symmetrical 
relations displayed in the gametic ratio. In order to show that the two fac- 
tors are linked, in this case we represent the genetic constitutions of the 
parents as {CW){CW), purple starchy, and {cw){cw), white waxy; not 
CCWW and ccww respectively, which is the form used to indicate inde- 
pendent relations between the factors. Correspondingly the Fi is 
{CW){cw), not CcWw, and the series of gametes which it forms is written 

3.4(CTF) :l{Cw) : l(cTF) :3.4(cw;). 

The method of deriving an F2 ratio from such a gametic series is shown 
in the checkerboard in Fig. 49. Here it is necessary to take into account 
not only the genetic constitutions of the gametes, but also the coefficients 
which represent their relative frequency. 

Summing up the totals for like phenotypes from this checkerboard, we 
find the F2 grains are distributed in the following ratio : 

50.28 with purple aleurone and starchy endosperm 
7.8 with purple aleurone and waxy endosperm 
7.8 with white aleurone and starchy endosperm 

11.56 with white aleurone and waxy endosperm. 
The calculated results based on this ratio are given in Table XIX. They 
show very close agreement with numbers actually observed, but in 
judging the significance of this agreement it must be borne in mind that 
a gametic ratio was arbitrarily selected which would give the closest 
possible agreement with the observed results. 



LINKAGE RELATIONS IN MEN DELI SM 



107 



When the factors enter the hybrid in (hfferent relations, the segre- 
gation ratio is different. Thus when purple waxy, {Cw){Cw), is crossed 
with white starchy, {cW){cW), the Fx is purple starchy as in the previous 
case. The resemblance, however, is not complete except as to phenotypic 

3.4 {CW) 1 {Cw) 1 {cW) 3.4 {cw) 



3.4 {CW) 



1 {Cw) 



1 {cW) 



3.4 {cw) 



11.56 

{CW){CW) 
purple starchy 


3.4 

{CW){Cw) 

purple starchy 


3.4 

{CW){cW) 
purple starchy 


11.56 

{CW){cw) 
purple starchy 


3.4 

{Cw){CW) 
purple starchy 


1 

{Cw){Cw) 
purple waxy 


1 

{Cw){cW) 

purple starchy 


3.4 

{Cw){cw) 
purple waxy 


3.4 

{cW){CW) 

purple starchy 


1 

{cW){Cw) 
purple starchy 


1 

{cW){cW) 
white starchy 


3.4 

{cW) {cw) 
white starchy 


11.56 

{cw){CW) 
purple starchy 


3.4 

{cw){Cw) 
purple waxy 


3.4 

{cw){cW) 
white starchy 


11.56 

{cw) {cw) 
white waxy 



Fig. 49. — Fi checkerboard of cross between purple starchy and white waxy maize. 



1 {CW) 


3.4 {Cw) 


3.4 (cTF) 


1 {cw) 


1 

{CW){CW) 
purple starchy 


3.4 

{CW){Cw) 
purple starchy 


3.4 

{CW){cW) 
purple starchy 


1 

{CW){cw) 

purple starchy 


3.4 

{Cw){CW) 

purple starchy 


11.56 

{Cw){Cw) 
purple waxy 


11.56 

{Cw){cW) 
purple starchy 


3.4 

{Cw) {cw) 

purple waxy 


3.4 

{cW){CW) 
purple starchy 


11.56 

{cW){Cw) 
purple starchy 


11.56 

{cW){cW) 
white starchy 


3.4 

{cW){cw) 
white starchy 


1 

{cw){CW) 

purple starchy 


3.4 

{cw){Cw) 
purple waxy 


3.4 

{cw){cW) 
white starchy 


1 

{cw) {cw) 
white waxy 



1 {CW) 



3.4 {Cw) 



3.4 {cW) 



1 {cw) 



Fig. 50. — F2 checkerboard of cross between purple waxy and white starchy maize. 

expression, for its genetic constitution is {Civ){cW), instead of (CW){cw) 
as in the first cross. It produces a series of gametes in the ratio 

1{CW) :SA{Cw) :3.4(cF) :l(,cw). 
In this ratio the numerical proportions of the gametes are reversed. 
This is due to the fact that here the original factor combinations, C and 



108 GENETICS IN RELATION TO AGRICULTURE 

w, and c and W, although the reverse of those in the former case, tend 
to remain together in the same ratio. 

When Fi plants of the genetic constitution, {Cw)(cW), are selfed 
segregation occurs in F2 as shown in the checkerboard in Fig. 50. When 
like phenotypes are collected into classes, the following distribution is 
obtained : 

39.72 with purple aleurone and starchj^ endosperm 
18.36 with purple aleurone and waxy endosperm 
18.36 with white aleurone and starchy endosperm 
1.00 with white aleurone and waxy endosperm. 

This ratio is strikingly different from that obtained for the former cross, 
although exactly the same characters are involved. Unfortunately data 
supporting this part of the analysis have not yet been presented in a 
satisfactory manner, but the results so far as reported do show a positive 
linkage between the factors. Moreover other cases which we shall discuss 
in this chapter demonstrate beyond doubt that the relations described 
above hold rigidly for cases of factor linkage. The different results 
obtained when factors enter a cross in different combinations are, there- 
fore, simply due to the fact that the original combinations tend to be 
preserved in segregation in a definite fixed proportion of gametes. 

To give a chromosome interpretation of linkage we assume that the 
factors linked are borne in the same chromosome. Thus the factor for 
purple aleurone color is one of the chromomeres occupying a definite 
locus in a particular pair of chromosomes in a purple starchy race of 
corn and the factor W for starchy endosperm occupies a different locus 
in these same chromosomes. In Fig. 51 the chromosome behavior in 
linkage is shown graphically. In the hybrid one member of a pair of 
chromosomes bears the factors C and W, the other member c and w. 
During synapsis these chromosomes conjugate, and when the threads 
representing the two chromosomes separate after conjugation they may 
in consequence of their twisted condition break at certain points and, 
reuniting, the free ends of different threads may join together. In a cer- 
tain percentage of cases this breaking of the filaments may occur between 
C and W, so that the chromosomes afterward reconstituted will contain 
the factors C and w, and c and W rather than the original combinations. 
More frequently the chromosomes will untwist without exchanging chro- 
matin material or after having exchanged it in such a way as not to dis- 
turb the original factor combinations. Exchange of chromatin material 
between homologous chromosomes is called crossing-over. This term 
is also applied to the formation of new combinations of linked factors, 
and these new combinations are called cross-overs. In this particular 
case the end result is that for the factors C and W and their allelomorphs 



LINKAGE RELATIONS IN MEN DELI SM 



109 



crossing-over occurs in 22.6 per cent, of cases. Accordingly the gametes 
are formed in the ratio : 

38.7 percent. {CW) : 38.7 per cent, (cw) : 11.3 per cent. (Cw) :11.3 percent. (cTT) 

77.4 per cent, non-cross-overs 22.6 per cent, cross-overs. 

It follows, therefore, that linkage may be interpreted as due to as- 
sociation of factors within the same chromosomes and that crossing-over 
or breaking apart of linked factors may be regarded as a consequence of 








I 



Fig. 51.— Diagrammatic representation of crossing-over and results. At the left, the 
two original chromosomes. In the middle, the twisted condition of the chromosomes in 
synapsis and their subsequent Reparation. At the right, the four types of chromosomes 
which result and their proportions. 






Fig. 52. — Diagrammatic representation of crossing-over and its results when the factors 
enter in the opposite combination from that shown in Fig. 51. 

chromatin exchange between homologous chromosomes during synapsis. 
The factors may be thought of as the purely passive objects with which 
the chromosome mechanism deals, they are Hnked together because they 
are borne in the same chromosome, they show breaks in linkage in a 
certain percentage of cases because in synapsis breaks occur between the 
loci which they occupy in the chromosome such that new combinations of 
the factors are formed. The chromosome relations are the same even 
when chromatin interchange results in no new combinations of factors, 



110 GENETICS IN RELATION TO AGRICULTURE 

it is only when there are factor differences between the homologous chro- 
mosomes that the operation of the mechanism can be detected and some 
conception gained of its mode of operation. 

Linkage in Drosophila. — To Morgan and his associates through 
their investigations with mutations of Drosophila aynpeloplila we owe 
directly practically our entire conception of the linkage relations dis- 
played by factors. No other single species has provided such a wealth 
of data or proved so favorable for genetic investigations. This body of 
data is still growing very rapidly and is adding new conceptions all the 
time, but even at this time it is no exaggeration to say that the Zoological 
Laboratories of Columbia University, like the old garden of the Konigs- 
kloster at Brtinn, have yielded results which will be accounted among 
the epochal advances in genetics. Mendel's work showed that the char- 
acters of the organism were dissociable elements of its makeup which 
could be recombined and shuffled about in genetic experiments. From 
this starting point the factor conception of heredity, which assumes that 
characters of the individual may be referred to the action of definite 
factors in the hereditary material, was developed by a host of investi- 
gators. Morgan's work has also furnished an overwhelming body of 
evidence supporting the factor conception of heredity, but its most im- 
portant contribution to genetics has been in the establishment of the 
relations existing between the factors of heredity and the chromosome 
mechanism of the cell. 

The Four Groups of Factors in Drosophila. — According to the chro- 
mosome theory of heredity a factor is located at a particular locus in the 
chromosome mechanism. Consequently since linkage depends upon 
factor relations within the same chromosome it follows that the factors 
should display linkage relations such that they would be thrown into 
groups corresponding to the number of pairs of chromosomes. In Droso- 
phila the linkage relations existing among over a hundred factor mu- 
tations have been studied. The factors fall into four groups correspond- 
ing to the four pairs of chromosomes in Drosophila, and furthermore the 
relative sizes of these groups corresponds roughly to the relative sizes 
of the different pairs of chromosomes. There is a large group of sex- 
linked factors all of which display the same type of inheritance as white- 
eye color, which has already been described. This group corresponds 
to the X-chromosomes. There are two large groups of factors which 
correspond to the two large pairs of autosomes, and finally there is a small 
group, consisting as yet of only two factors, which corresponds to the 
small pair of autosomes. The following list of the groups of factors 
in Drosophila, although incomplete, gives some idea of the number and 
kinds of factors which have been studied in this species (Table XX). 

The type of behavior shown in linkage in Drospohila may be illus- 



i 



LINKAGE RELATIONS IN MEN DELIS M 



111 



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a ^ 



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-a i»i -^c ic r^ u •►-* it; -G >> >i 'C >) 'iT >> o •►r; o ^ -^ x^ a >>'>^ ° s s &►' ^ i; i. ,'?,'? i; 



'^ (y d C O ^ 



a» o o o o o 

— - — o o •►" >. >i o 



s 



h a -^ 



a; ^ t. 0) a <u C 



o o3 .2 






^ S o o o 
J3 J3 o. a , 



a a .-5 ° 
Eh > ^ >^ 



112 



GENETICS IN RELATION TO AGRICULTURE 



trated by the cross yellow white female by gray red male. The genetic 
formula of a yellow white female is {ywX){ywX), and of a gray red 
male {YWX)Y. In these formulae y stands for the factor for yellow 
body-color and w for white eye-color, and the capital letters for the corre- 
sponding dominant allelomorphs present in the wild type. When two 
such flies are bred together the Fi consists of females of the genetic con- 
stitution {YWX){ywX) and males of the genetic constitution {ywX)Y. 
The Fi females, therefore, have gray bodies and red eyes and the males 
have yellow bodies and white eyes. Gray red females of the genetic 
constitution {YWX){ywX) produce four kinds of eggs in the following 
proportions : 



{YWX) 49.45 per cent. (ywX) 49.45 per cent. 



5.9 per cent, non-cross-over gametes. 



(YwX) 0.55 per cent. (yWX) 0.55 per cent. 
1.1 per cent, cross-over gametes. 



When such a female is bred to a yellow white male, genetic constitu- 
tion {ywX)Y, which produces only two kinds of sperms, (ywX) and Y, 
the progeny in both sexes obviously will be in the ratio 

49.45 gray red: 49.45 yellow white: 0.55 gray white: 0.55 yellow red. 
Table XXI shows the results which have been secured, mostly from matings 
of this type. 

The relations shown when the factors enter in the reverse combinations 
may be determined by mating a gray white female, genetic constitution 
{YwX)(YwX) to a yellow red male, genetic constitution (yWX)Y. 
This gives F^ gray red females of the constitution {YwX)(yWX), and 
gray white males of the genetic constitution {YwX)Y. In this case 
the Fi females produce four kinds of eggs in the following proportions: 



{YwX) 49.45 per cent. : (yWX) 49.45 per cent. 
98.9 per cent, non-cross-ever gametes. 



(.YWX) 0.55 per cent. -.{ywX) 0.55 per cent. 
1.1 per cent, cross-over gametes. 



Table XXI. — Factor Linkage in Gray Red Female Drosophilas of the Type 

{YWX)(ywX) 





Number 

of flies 

classified 


Non-cross-overs 


Cross-overs 


Percentage 


Reported by 


Gray Yellow 
red white 


Gray 
white 


Yellow 
red 


of cross- 
ing-over 


Dexter 


14,939 

1,818 

854 

3,424 


8,093 6.672 


93 
14 

2 
7 


81 

5 

10 


1.16 


Morgan and Cattell 

Morgan and Cattell 

Morgan and Bridges .... 


1,075 

513 

1,807 


729 

334 

1,600 


0.77 
0.82 
0.50 


Totals 


21,035 


11,488 


9,335 


116 


96 


1.01 







LINKAGE RELATIONS IN MEN DELI SM 



113 



When, therefore, the F\ females are mated to yellow white males of 
the genetic constitution (ywX) Y the progeny will give directly in its 
phenotypic ratio the proportions in which the gametes are produced as 
follows : 

49.45 gray white: 49.45 yellow red: 0.55 gray red: 0.55 yellow white. 
The actual experimental results from this type of mating are summarized 
in Table XXII. The actual linkage value obtained is 1.13 which is substan- 
tially the same as that shown in the previous table. On the basis of sum- 
marized data of counts of 81,299 flies, Morgan and Bridges fix the value 
for crossing-over between these two loci at 1.1 per cent. This is the value 
we have used in deriving the above gametic ratios. Other factors have 
been studied in the same way and give different percentages of crossing- 
over. Thus Morgan and Bridges report the value for crossing-over 
between white and miniature based on counts of 110,701 flies at 33.2 
per cent., between white and vermilion from 27,962 flies at 30.5, between 
vermilion and bar from 23,522 flies at 23.9, and so on for the whole series 
of factors in the first group. 



Table XXII. — ^Factor Linkage in Gray-red Female Drosophilas of the Type 

{YwX){yWX) 





Number 

of flies 

classified 


Non-cross-overs 


Cross 


-overs 


Percentage 


Reported by 


Gray 
white 


Yellow 
red 


Gray 
red 


Yellow 
white 


of cross- 
ing-over 


Dexter 


1,348 
3,258 
9,027 

13,633 


440 
1,841 
4,292 


889 
1,412 
4,605 


16 
4 

86 


3 
1 

44 


1.41 


Morgan and Cattell 

Morgan and Cattell 


0.15 
1.44 


Totals 


6,573 


6,906 


106 


48 


1.13 



Factors in the second and third groups display the same type of 
linkage relations as those in the first group. As an example we may take 
the recessive factors black and curved which lie in the second group. A 
black curved female of the genetic constitution (6c„)(6ct,) crossed with 
a gray normal male (BCv) (BCv) gives in Fi gray long females and males 
of the genetic constitution (BCv)(bCv). When such Fi females are 
crossed back to black curved males the results as reported by Sturtevant 
and Bridges are given in Table XXIII. The observed percentage of 
crossing-over between the loci B and Cv in the second chromosome in 
this experiment amounts to 24.04 per cent. 

When gray curved females {Bcv){Bcv) are mated to black normal 
males (6C^)(6C„) the Fi flies are gray normal as in the previous case, 
but genetically they are of the constitution (Bc„)(6C»). Such females 



114 



GENETICS IN RELATION TO AGRICULTURE 



mated to black curved males, according to the same investigators, 
gave the results tabulated in the last two columns in Table XXIII. Here 
the percentage of crossing-over amounted to 22.74 per cent., a value 
substantially in agreement with the results of the reverse factor tests. 

Table XXIII. — Crossing-over Between B and Cv in Drosophila 



Phenotype 



Black curved cf (bcr) (bcv) mated to 



Gray normal 9 (BC„)(6c„) 



Non- 
cross-overs 



Cross-overs 



Gray normal $ (Sc„) (bCv) 



Non- 
cross-overs 



Cross-overs 



rGray normal 

Gray curved 

Black normal 

Black curved 

Totals 

Percentage of crossing-over 



610 



652 



184 
226 



2,292 
2,148 



644 



663 



1,262 



410 



4,440 1,307 



24.04 



22.74 



No Crossing-over in the Male. — The above results show clearly that 
crossing-over in the female between the loci B and C„ of the second chro- 
mosome results in the production of approximately 23 per cent, of cross- 
over gametes irrespective of the particular combination of the factors 
concerned. Sturtevant and Bridges, however, have shown that there is 
no crossing-over in the male so that males of the genetic constitution 
{BCv)(bCv) produce only two types of sperm in the ratio l{BCv) :l(6c„) 
and males of the genetic constitution (Bc„)(bCv) produce sperms in the 
ratio, l(5c„) :1(6C,). 

It is not known just what the absence of crossing-over in the male 
depends upon. In the case of factors in the X-chromosome or first 
group, crossing-over would involve exchange of chromatin material be- 
tween the X- and F-chromosomes. Since these differ strikingly it is 
not surprising that interchange of chromatin does not take place between 
these chromosomes for it is difficult to see how the difference could be 
preserved if crossing-over should occur. But the other chromosomes are 
alike in both sexes, nevertheless no matter how high the percentage of 
crossing-over in the female, none whatever has been observed in the male. 
This has been found true for factors lying in the third group as well as 
for those lying in the second group, and it is without doubt a general 
phenomenon. 

The knowledge that no crossing-over occurs in the male has often 
been turned to advantage in experimental work. When gray curved 
flies, {Bcv){Bcv) are mated to black normal^ {bCv)(bCv), the Fi flies are 



LINKAGE RELATIONS IN MEN DELI SM 



115 



gray normal and of the genetic constitution {Bcv)(bCv). When these are 
interbred to obtain the F2 generation the results are shown in the checker- 
board in Fig. 53. According to this checkerboard, the F2 will consist of 
flies in the ratio 

2 gray normal : 1 gray curved : 1 black normal. 

No black curved flies are obtained in this cross in F2, and it is of interest to 
note that no matter what the amount of crossing-over in the female the 
flies in F2 will always be in the ratio 2:1:1. In F3 black curved flies 
may be obtained from a certain percentage of matings of black normal 
or of gray cvu'ved flies. 

The failure of the double recessive class to appear in F2 has been much 
used by Morgan in determining the factor group to which new mutations 

(Be) (bC) 



11.5 (BC) 



11.5 

(fiC„)(Sc„) 
gray normal 


11.5 

{BC,){bC,) 
gray normal 


38.5 

(Be.) (Be,) 
gray curved 


38.5 

(Bc„)(6C.) 

gray normal 


38.5 

(.bC.){Bc„) 

gray normal 


38.5 

(6C.)(6C.) 

black normal 


11.5 

(be,,) {Be.) 
gray curved 


11.5 

{bC,){bC^) 

black normal 



38.5 {Be,) 



38.5 {bCv) 



11.5 (be,) 



Fig. 53. — F2 obtained by crossing gray curved and black normal flies. 

belong. For this purpose black-pink flies are crossed with the new mutant 
type. Since black lies in the second group and pink in the third, if the 
new factor belongs to either of these groups it will fail to show the corre- 
sponding double recessive form in F2. Whether it belongs to the sex- 
linked group is of course readily determined from the sex relations ob- 
tained in such an experiment, and if the test shows that the factor in 
question belongs to none of these three groups, by exclusion it must be- 
long to the fourth. 

Linear Arrangement of Factors. — It was an old idea of Roux brought 
forward to explain the division of the chromatin while in the form of along 
thin thread that the individual elements of the chromatin are arranged 
in a linear series in the chromosomes. Later Janssens developed the 
idea that in synapsis homologous chromosomes twist about each other 



116 



GENETICS IN RELATION TO AGRICULTURE 



and in separating tend to break apart at places, and in reuniting exchange 
chromatin material. Morgan has taken these two ideas and applied 
them to the results of the Drosophila investigations. The twisting of the 



0.0 ! Yellow, Spot 

1.1 j White, Cherry, Eosin 
2.4 Abnormal 



6,3 



Bifid 



12.5 Lethal H 
14.6 ' Lethal sb 
16.7 Club 



19.5 

23.7 
26.5 



Lethal Ilia 

Lethal sa 
Lethal III 



33.0 Vermilion 

36.1 Miniature 
38.0 Furrowed 



43.0 



55.1 
57.0 
59.5 



66.2 



Sable 



Radimentary 

Bar 

Fused 



Lethal sc 



16.6 



Streak 



Dachd 



34.7 Black 
35.0 



40.0 



52.0 



60.4 



Jaunty 
Purple 



Vestigial 



Curved 



84.1 Arc 



90.0 
91.9 



Speck 
Morula 



0.0 



Sepia 



Bent 

Eyeless 



25.0 Pink, Peach 



40.0 Kidney 



55.0 Ebony, Sooty 



73.0 Beaded 



85.0 



Rough 



Fig. 54. — Plotted relations of factors in Drosophila ampelophila. 



chromosomes in synapsis is held to be the physical evidence of the known 
interchange of factors between chromosomes which takes place in crossing- 
over. Moreover, if the factors are the individual elements of the chro- 



LINKAGE RELATIONS IN MEN DELI SM 117 

matin threads which twist about each other and these elements are held 
to occupy invariable loci in the chromatin thread, then the percentage of 
crossing-over between any two loci may be taken as an indication of the 
distance between the factors. For obviously if the chromatin thread is 
as likely to break between any two chromomeres as between any other 
two, then the farther apart two factors lie in the chromatin threads repre- 
senting homologous chromosomes, the greater is the chance that crossing- 
over will occur between them. 

The results of the application of this idea to the linkage relations 
existing in Drosophila are shown in Fig. 54. In this chromosome map of 
Drosophila the factors have been plotted in a linear series according to 
their relative position in the chromosomes as determined by linkage rela- 
tions. The evidence as yet is not sufficient to give an accurate picture of 
the arrangement of all the factors, but the number of factors plotted and 
the relations which they display provide further evidence of the corre- 
spondence between the chromosomes and the factor groups. Morgan 
has taken 1 per cent, of crossing-over as the unit for expressing linkage 
relations. Expressed in such units the first chromosome, which contains 
all the sex-linked factors, has a length of 66.2. The second and third 
groups, as far as determined, have lengths of 91.9 and 85.0, respectively. 
These lengths in general correspond fairly well to the known relative 
sizes of the two large pairs of autosomes when compared with each 
other and with the X-chromosomes. In the fourth group but two 
factors are known and their loci are so close together that thus far no 
crossing-over has been observed between them. Accordingly no definite 
value can be fixed for their linkage relations. From a knowledge of 
the small relative size of the third autosome Muller, at the time he an- 
nounced the discovery of the first factor in the fourth group, predicted 
that factors in this group would show very close linkage values. This 
prediction has been upheld satisfactorily and it is further evidence that 
the chromosome theory of heredity works. 

The demonstration that factors lie in a linear series in each group 
provides a unique method of predicting the results of factor behavior. 
Obviously if a factor is known to belong to a particular group, it is 
possible to predict confidently that it will display independent segre- 
gation with factors belonging to other groups. But further than this 
when the loci of a number of factors in a given group have been plotted 
accurately, with a new factor it is only necessary to determine the linkage 
relations with two of the plotted loci in order to determine its locus. 
When its locus has been determined, its linkage values with any other 
members in the group may be predicted from its distance in units from 
those factors. To illustrate, in Group I, if the position of miniature 
were unknown, it might be tested with vermilion and sable. It would 



118 



GENETICS IN RELATION TO AGRICULTURE 



give about 3 per cent, of crossing-over with vermilion and about 7 per 
cent, with sable. Knowing the position of the vermilion locus at 33.0 
and the sable locus at 43.0, we would be able from these data to fix the 
locus position of miniature at about 36.0. With this value determined 
we could confidently predict for example that miniature and white 
would show somewhat less than 35 per cent, of crossing-over or miniature 
and bar about 21 per cent. The ability to make such predictions is a 
unique product of recent investigations in heredity. 

How experimental results support the hypothesis of linear arrange- 
ment of factors may be illustrated by what Morgan calls a three-point 
experiment, i.e., an experiment involving three different factors in the 
same chromosome. We may take three factors which are in widely 
separated loci in the chromosome, white at locus about 1.0, miniature 
at about 36.0, and bar at about 57.0. The summaries which Morgan 
and Bridges have given of the data involving these three loci are in- 
cluded in Table XXIV. White and miniature give directly 33.2 per cent, 
of crossing-over, and miniature and bar 20.5 per cent. Since the distance 
between white and miniature plus that between miniature and bar is 
equal to 53.7, this latter value should represent the distance between 
white and bar. But direct experimental determinations of the per- 
centage of crossing-over between white and bar give a value of 43.6 per 
cent., which is 10.1 per cent, short of the calculated value. 

Table XXIV. — Crossing-over for the Loci W, M, and B' in Drosophila. 

Data Obtained by Mating Females op the Constitution 

{wmB'X){WMh'X) With Triple Recessive Males (wmb'X)7. 



Character combinations 


Number 

of flies 

classified 


Number 
of cross- 
overs 


Per cent, 
of crossing- 
over 


White miniature. . . . 

Miniature bar 

White bar 


110,701 
3,112 
5,955 


31,071 

636 

2,601 


33.2 
20.5 
43.6 







The reason for this should be plain from a consideration of Fig. 55 
which shows diagrammatically how the chromosomes behave in a three- 
point experiment. On the left in the two upper groups are represented 
the two chromosomes with the factors in the original positions in which 
they were derived from the parents. On the right the homologus chromo- 
somes are shown twisted about each other, and dit A, B, C, and D the 
types of chromosomes which are obtained after chromatin interchange in 
synapsis. The numbers below refer to the relative frequency of pro- 
duction of the four types of chromosome pairs in this three-point experi- 
ment based on the data of Table XXIV. In A (Fig. 55) no exchanges 



LINKAGE RELATIONS IN MEN DELI SM 



119 



of chromatin have occurred which affect the relations of the factors to 
each other, so that this type of separation after synapsis gives the non- 
cross-over gametes {wmB'X) and {WMh'X). Types B and C involve 
single breaks in the chromosomes followed by chromatin interchange in 
reunion. They are the single cross-overs and give the cross-over gametes 



/'-\ 



B 




46.3 



28.15 



15.45 



10.1 



Fig. 55. — Diagram to show crossing-over in a three-factor experiment. 



{wMh'X) and {WmB'X) and {wmb'X) and (WMB'X) respectively. 
Finally in type D the chromosomes have broken and exchanged material 
at two points. This type is called double crossing-over and results in 
the production of gametes of the genetic constitutions {wMB'X) and 
(Wmh'X). In this last case, although chromatin interchange has 
occurred between the two chromosomes, the relations between the loci 
W and B' remain unchanged. 



120 GENETICS IN RELATION TO AGRICULTURE 

The occurrence of double crossing-over accounts for the low per- 
centage of crossing-over between white and bar as compared with the 
sum of the values given by white and miniature and miniature and bar. 

The value for crossing-over between W and M is given by 

B -\-^ = 28.15 + 5.05 = 33.2 per cent. 

and similarly between M and B' 

C + ~ = 15.45 + 5.05 = 20.5 per cent. 

consequently the distance between W and B' as measured by adding 
together the values for W and M and M and B' gives the equation 

B -\- C -\- D = 53.7. 

Since double crossing-over of the type D does not involve a rearrange- 
ment of the loci, W and B', however, the actual crossing-over obtained 
experimentally must fall short of the computed distance by a value 
equal to D as given by the equation 



B + C = 43.6 per cent. 



The lowering of the percentage of crossing-over when extreme distances 
are involved is, therefore, a logical consequence of the relations existing 
between linked factors. Obviously double crossing-over occurs much less 
frequently in short distances than in long ones. Consequently since a 
factor map is designed to give the total values for crossing-over between 
the different loci, such a map is prepared so far as possible from experi- 
ments involving short factor distances. If such data are not at hand 
simple methods of interpolation are used to locate the loci. 

It should be noted in passing that variations in linkage values some- 
times occur among members of a given set of factors. Bridges has pointed 
out that in some cases at least the percentage of crossing-over depends 
somewhat on the age of the female, and Plough has detected definite 
effects of extremely high or low temperatures on the percentage of 
crossing-over between factors of the second chromosome in Drosophila, 
although crossing-over in the first and third chromosomes was not in- 
fluenced by the changes in temperature. Besides such variations, how- 
ever, definite factors have been discovered (Sturtevant) which lower the 
percentage of crossing-over. Muller has shown that such a factor exerts a 
particularly disturbing action in the third chromosome in which it is 
located. But even in cases of variation in linkage values the order of the 
factors in the chromosome is not disturbed. The relations shown, there- 
fore, in cases involving variations in linkage are in harmony with the 
conception of linear arrangement of factors in the chromosomes. 



LINKAGE RELATIONS IN MEN DELI SM 121 

The most striking confirmation of the hypothesis of linear arrange- 
ment is found in the case of "deficiency" in the A'^-chromosome, which 
was investigated by Bridges (see p. 155) and in which the location of 
forked spines within the deficient region "was detected and proved as a 
result of deliberate search among those genes which had previously been 
mapped closest to bar!" 

The Mode of Interchange in Crossing-over. — Factor interchange 
conceivably might take place by interchange of isolated factors here and 
there along adjacent threads or it might follow as a consequence of inter- 
change of relatively large sections of chromatin between chromosomes. 
The sectional mode of chromatin interchange appears to have more cyto- 
logical evidence in its support and Plough's recent studies on the effect 
of temperature on crossing-over corroborate Muller and Bridges' inference 
that crossing-over takes place in the fine thread stage of synapsis, which 
would be the most favorable stage for sectional interchange. But breed- 
ing investigations of themselves clearly establish this hypothesis. Thus 
Muller made up females which contained twelve sex-linked mutant 
factors. These females received from one parent the factors for yellow 
body color, white eye color, abnormal abdomen, bifid wings, vermilion 
eye color, miniature wings, sable body color, rudimentary wings, forked 
spines, and from the other parent the mutant factors cherry eye color, 
club wings and bar eyes. Using the system of writing the genetic 
formulae which has been followed in this text, these females were of the 
genetic constitution 

(ywA%Civmsrfb'X)iYw<=a'BiCiVMSRFB'X). 
Muller found in tests of 712 individuals arising from gametes from such 
females, that the proportions of crossing-over between factor loci in the 
formation of gametes occurred according to the figures given in Table 
XXV. The results show that in this experiment there was no crossing-over 
in 54.4 per cent, of cases; single crossing-over in 41.7 per cent., and double 
crossing-over in 4.2 per cent. No example of triple crossing-over was 
found among these flies, but a few such cases have been observed. The 
values agree satisfactorily with those calculated from the three-point 
experiments involving the loci W, M, and B' in this same chromosome. 
If we consider the double cross-overs which were obtained in this 
experiment we find abundant evidence in support of the sectional mode 
of chromatin interchange. It is difficult to visualize the relations from 
the numerical data, consequently Fig. 56 has been prepared to illustrate 
diagrammatically the types of double crossing-over obtained in these 
experiments. In all but one case the points of crossing-over are far re- 
moved from each other, and even in the exceptional case the distance 
between the points of crossing-over may have been as great as nineteen 
units distance. 



122 



GENETICS IN RELATION TO AGRICULTURE 



Table XXV.— Classification of Factor Combinations Transmitted by Females 
OF Drosophila having the Genetic Constitution 

{ywA'biCivmsrfb'X)iYvfa'BjCiVMSRFB'X) 



No crossing-over 



186 



200 



386 



Crossing-over between the loci 



Number of 
yellow flies 



Number of 
gray flies 



Totals 



Yellow and white 

White and abnormal 

Abnormal and bifid 

Bifid and club - 

Club and vermiUon 

Vermilion and miniature 

Miniature and sable 

Sable and rudimentary 

Rudimentary and forked 

Forked and bar 

Total single cross-overs 

Double crossing-over 

Y and W:Ci and V 

YandW-.M and S 

Y and W:S and R 

Y and W:R and F 

TF and A' : Cj and F 

W and A' : R and F 

A' and Bi-.C I and V 

A' and Bi : S and R 

Bi and Ci : M and S 

Bi and Ci : S and R 

Ci and 7: 7 and M 

Ciand V:SandR 

Ci and V : R and F 

Ci and V : F and B' 

Total double cross-overs 



2 
3 

4 
17 

46 

7 

18 

28 



5 

5 

11 

27 

51 

9 

19 

38 

5 

1 



15 
44 
97 
16 
37 
66 
5 
1 



296 




Totals 



30 



Interference. — Interference is merely a consequence of the sectional 
mode of chromatin interchange between homologous chromosomes. The 
term is used to designate the observed fact that when crossing-over takes 
place at a particular point in the chromosome the regions for some dis- 
tance on both sides are protected from coincident crossing-over. The 
operation of interference is well illustrated in Muller's data, although 
the numbers are not sufficient to warrant a quantitative determination of 
its effect. With long distances interference decreases, which is in accord- 
ance with expectation. Even for relatively long distances, however, as 
for the loci W, M and B' which we have already considered in detail 



LINKAGE RELATIONS IN MEN DELI SM 



123 



there is still some evidence of interference. Based purely on the laws 
of chance, if crossing-over occurs between W and M in 33.1 per cent, of 
cases and between 71/ and B' in 20.5 per cent., then the chance of coinci- 
dent crossing-over is equal to the product of the independent chances of 
crossing-over. This gives a value of 6.8 per cent, which is slightly greater 
than the value 5.05 per cent, calculated from the experimental data. 

A three-point experiment involving shorter distances, however, gives 
a clearer idea as to the extent of interference. Morgan and Bridges 






35 A 
5 5 B, 



29-V 
31 -M 






I I 

48.5R I 

I SO B^ 



I I 



I 



I 



i 




I 



I I 



I 



I 



i 



i 

11 21 1111271821 

Fig. 56. — Diagram showing types of double crossing-over in females of Drosophila 
heterozygous for twelve sex-linked factors. The figures below indicate the number of times 
the type occurred in 712 cases. (The loci indicated in the " map" at the left are only approx- 
imately correct according to recent data of Morgan and Bridges, but they are sufficiently 
accurate for the purpose of this diagram.) 



have reported such an experiment involving the loci for vermilion, sable, 
and bar with the results given in Table XXVI. From this table the total 
percentage of crossing-over between vermilion and sable is 9.8 per cent, 
and between sable and bar 13.8 per cent. The expected percentage of 
double crossing-over for these values obtained by taking 9.8 per cent, of 
13.8 per cent, would be 1.35 per cent. The observed amount of double 
cro.ssing-over, 0.25 per cent., is only about one-fifth of this value. 

That interference is normally to be expected from the method of 
chromatin interchange in synapsis may be seen clearly by a consideration 
of Fig. 57. Thus if the chromosomes have a modal length in loop 
twisting about each other in synapsis, then a crossing-over at point B 



124 GENETICS IN RELATION TO AGRICULTURE 

Table XXVI. — ^Linkage of Vermilion, Sable, and Bar in Drosophila 



'■ 


Non- 
cross-overs 


Single cross-overs between 




Characters 


Vermilion and 
sable 


Sable and 
bar 


Double 
cross-overs 


Gray red normal 


755 

734 
724 
845 
608 
800 
665 
641 


110 
92 
97 

87 
80 
95 
81 
74 


140 
151 
131 
126 
123 
129 
107 
108 


4 


Gray vermilion normal 


1 


Sable red normal 


4 


Gray red bar 


4 


Gray vermilion sable 


3 


Gray vermilion bar 


1 


Sable red bar 


1 


Vermilion sable bar 


3 






Totals 


5,772 


716 


1,015 


21 


Percentages 


76.7 


9,53 


13.49 


0.28 












A D F 

Fig. 57. — Diagram to illustrate interference in crossing-over. 



would protect the loci A, C, and D on either side of it from crossing-over 
because there would be no close twisting of the chromosomes at these 
points. As we move on toward E, however, the frequency of double 
crossing-over would become greater and greater until at E where the 
modal length of loop was attained double crossing-over values approach- 
ing those expected on the basis of pure chance would be obtained. 
Muller has actually shown that such conditions are fulfilled in his 
twelve-point experiments and he has been able to plot a curve showing 
that the observed frequency .of double crossing-over gradually increases 
until when the modal length is reached the curve coincides with that 
based on pure chance. Thus we see again how another point of attack 
has lent support to the conception that the factors are arranged in a 
linear series and that the linkage relations of factors are referable to the 
mechanical consequences of relative positions in the linear series. 

Bridges points out that interference stands in about the same relation 
to linkage as linkage does to free Mendelian assortment. Also that the 
development of the idea of interference is an illustration of the advantage 
of the chromosome hypothesis. The existence of this phenomenon was 



LINKAGE RELATIONS IN MENDELISM 125 

originally deduced by Miiller and Sturtevant from a consideration of 
linkage as a chromosome hj'pothcsis. 

Linkage Phenomena in Other Plants and Animals. — Our extended 
discussion of linkage relations has been based practically entirely on 
Drosophila anipelophila because the factor analysis in this species pro- 
vides us with a body of data incomparably superior to that provided 
by any other species. Nevertheless there are other scattered cases of 
linkage in many species of plants and animals. 

In plants Bateson first described the phenomenon of linkage in sweet 
peas for the characters round pollen and red flower color. Later the 
factor for hooded standard was found to be linked to the factors for these 
two characters. Later Punnett discovered linkage in a second group of 
factors consisting of those for green axils, cretin flower shape, and sterile 
anthers. Gregory has described a group of five factors in the Chinese 
primrose, those for red stigma, red flower color, long style, dark stems, 
and light corolla tube. In garden peas, Vilmorin and Bateson have both 
reported linkage between the factors for round as opposed to wrinkled 
seed and tendrilled as opposed to non-tendrilled or "acacia" leaves, and 
Hoshino has suggested coupling between red flower color and a factor 
for late flowering. Very recently O. E. White has investigated or com- 
piled the data on thirty-five factor differences in Pisum and has presented 
data for four linked groups of characters. In the garden snapdragon, 
Baur found linkage between the factors for red flower color and for the 
"picturatum" type of color pattern in the flower and also clear cut 
evidence of linkage between some other factors. Surface has shown that 
in oats the factors for pubescence on the back of the lower grain, pu- 
bescence on the back of the upper grain, and black grain color are closely 
linked. Enough cases have also been reported for other plants to demon- 
strate that linkage relations are of general occurrence in plants. 

In animals, Castle and Wright have suggested that linkage occurs in 
rats between the factors for red eye-color and pink eye-color. A clear 
case also has been established by Tanaka in the silkworm moth in which 
a series of factors for larva pattern are linked to factors for yellow and 
white cocoon color. Besides these cases there are a large number of cases 
of sex-linked inheritance in many animal forms. These will be discussed 
in Chapter XI. 

It is clear from what we have stated above that aside from our 
knowledge of linkage in Drosophila, we have not progressed far in the 
investigation of linkage relations. Several factors have contributed to 
this condition. Most of the forms which have been used in genetic in- 
vestigations have a larger number of chromosomes than Drosophila, a 
fact which considerably complicates such investigations. Most genetic 
data have been obtained from experiments which involve but few fac- 



126 GENETICS IN RELATION TO AGRICULTURE 

tors. If the chromosome number is large, the chances" of such experi- 
ments showing factor Hnkage are sHght. Finally there are experimental 
difficulties in the way of securing an adequate body of data for most 
animals and for practically all plants. It is necessary to conduct most 
technical investigations in heredity with relatively meager financial sup- 
port, consequently the expenditures necessary to obtain sufficient data 
of this kind would be prohibitive for most of the larger animals and 
plants. Moreover, on account of the time required to raise a sufficient 
number of generations and to classify the individuals a considerable 
time must elapse before a body of data can be gathered in any species 
sufficient to submit it to the critical tests necessary to establish the 
chromosome theory. Drosophila with its prolific breeding tendencies, 
short life cycle, and ease of handling provides a form far superior to any 
other thus far investigated for the elucidation of factor relations in general. 
It is safe to say that our ideas of linkage for some time to come will be 
largely determined by the results of the Drosophila investigations. 
Particularly is this true because thus far none of the linkage phenomena 
exhibited by other animals and by plants have yielded evidence contradic- 
tory to the chromosome theory. The number of factors which have been 
investigated in several species exceeds the number of pairs of chromo- 
somes, nevertheless in no single case has there been a clear demonstra- 
tion that the number of independently Mendelizing factors exceeds the 
number of pairs of chromosomes. Moreover, those cases of linkage 
which have been discovered are largely of factors for wholly unrelated 
characters, just as in Drosophila. Added to this the ratios are of the same 
diverse orders of magnitude and the linkage relations in general show no 
essential difference from those which are displayed by Drosophila. It 
would be nothing short of inconceivable, in fact, that the conclusions 
reached from the Drosophila investigations are not applicable in all their 
essential features to plant and animal forms in general. 

On the basis of the sweet pea and Primula investigations, the English 
school of geneticists, represented particularly by Punnett and Trow, has 
developed a theory of linkage very different from that outlined in this 
chapter, which is called reduplication. According to this hypothesis 
segregation occurs in a series of cell divisions preceding the reduction 
divisions, and for linked factors gives gametic series mostly of the form 

For coupling (n — 1):1:1: (n — 1) 
For repulsion 1: (n — 1): (n — 1): 1. 

In these ratios n is some power of two. Interaction of two such series 
may give secondary reduplications which give different values for the 
terms of the ratio. This theory of linkage cannot, however, lay claim 
to the experimental support which the chromosome theory has obtained, 



LINKAGE RELATIONS IN MENDELISM 127 

nor is it based on any known cytological phenomena. The series of 
ratios which lent original support to the theory appear to be no more fre- 
quent than should be the case on the basis of chance, and many which are 
supposed to fall into the series have been placed there on evidence which 
is entirely inadequate. The large series of linkage values which have been 
obtained in Drosophila demonstrate clearly that all intermediate ratios 
can be obtained, and since all other conditions are satisfied by the chro- 
mosome theory it seems unreasonable to give it up for an hypothesis which 
has no cytological support and an uncertain amount of experimental 
support. Moreover, it may be safely stated that all cases of linkage 
thus far reported may be explained according to the chromosome theory 
of linkage. 

The mathematical relations existing in linkage phenomena are 
of interest because they provide a method of determining the genetic 
relationships involved in certain cases of somatic correlations. If two 
factors are linked in inheritance it follows that a larger proportion of 
the population will display the corresponding two characters than would 
be the case, if the factors were inherited independently. Consequently 
character correlations of this type are an index to factor linkage. 

In Tables XXVII and XXVIII the results of various strengths of 
factor linkage and the consequences with respect to the gametic and 
phenotypic ratios are given. 

These tables show clearly that the only satisfactory method of deter- 
mining the presence of linkage and its value is to cross back the hetero- 
zygous individual to individuals recessive for both factors. In such 
crosses the phenotypic ratio corresponds exactly to the gametic ratio, and 
it is, therefore, possible to determine the percentage of crossing-over by 
this method with a much greater degree of precision than from ordinary 
F2 populations. When the two dominant factors enter the cross from 
opposite sides it is practically impossible to determine the linkage 
values by simply mating Fi individuals together, for comparatively 
large differences in linkage value may affect the phenotypic ratio so 
slightly that the deviations, in small populations at least, might be 
ascribed merely to the operation of the laws of chance. The significant 
feature of such ratios is the small proportion of double recessives which 
appear. Thus with crossing-over values exceeding 20 per cent., this 
class practically disappears in experiments involving the usual number of 
individuals in a population. Moreover, such matings in species which dis- 
play crossing-over only in the sex-homozj^gotes as shown in Table XXVIII 
give the ratio 2:1:1 for all percentages of crossing-over when the domi- 
nant factors enter the cross from one parent only. A careful consider- 
ation of these two tables will show clearly how difficult it is to determine 
linkage values precisely except by properly planned experiments, and in 
this difficulty lies the reason for many errors of interpretation. 



128 GENETICS IN RELATION TO AGRICULTURE 

Table XXVII. — ^Linkage Relations — Crossing-over in Both Sexes 



Pl 


Percentage of 
crossing-over 


Gametic ratio 


Phenotypic ratio per thousand 


















AB:Ah:aB:ab 


AB 


Ah 


aB 


ah 




1 




99.0:99.0:1 


500.025 


249.975 


249.975 


0.025 




2 




49.0:49.0:1 


500.100 


249.900 


249.900 


0.100 




3 




32.3:32.3:1 


500.225 


249.775 


249.775 


0.225 




4 




24.0:24.0:1 


500.400 


249.600 


249.600 


0.400 




5 




19.0:19.0:1 


500.625 


249.375 


249.375 


0.625 


6 




15.7:15.7:1 


500.900 


249.100 


249 . 100 


0.900 


X 


7 




13.3:13.3:1 


501.225 


248.775 


248.775 


1.225 


.0 


8 




11.5:11.5:1 


501.600 


248.400 


248.400 


1.600 




9 




10.1:10.1:1 


502.025 


247.975 


247.975 


2.025 




10 




9.0: 9.0:1 


502.500 


247.500 


247.500 


2.500 




20 




4.0: 4.0:1 


510.000 


240.000 


240.000 


10.000 




30 




2.3: 2.3:1 


522.500 


227.500 


227.500 


22.750 




40 




1.5: 1.5:1 


540.000 


210.000 


210.000 


40.000 




50 


1: 1 : 1 :1 


562.500 


187.500 


187.500 


62.500 




40 


1.5:1:1 


1.5 


590.000 


160.000 


160.000 


90.000 




30 


2.3:1:1 


2.3 


622.500 


127.500 


127.500 


122.500 




20 


4.0:1:1 


4.0 


660.000 


90.000 


90.000 


160.000 




10 


9.0:1:1 


9.0 


702.500 


47.500 


47.500 


202 . 500 


A 


9 


10.1:1:1 


10.1 


707.025 


42.975 


42.975 


207.025 


e 


8 


11.5:1:1 


11.5 


711.600 


38.400 


38.400 


211.600 


X 


7 


13.3:1:1 


13.3 


716.225 


33.775 


33.775 


216.225 


05 


6 


15.7:1:1 


15.7 


720.900 


29.100 


29.100 


220.900 




5 


19.0:1:1 


19.0 


725.625 


24.375 


24.375 


225.625 




4 


24.0:1:1 


24.0 


730.400 


19.600 


19.600 


230.400 




3 


32.3:1:1 


32.3 


735.225 


14.775 


14.775 


235.225 




2 


49.0:1:1 


49.0 


740.100 


9.900 


9.900 


240.100 




1 


99.0:1:1 


99.0 


745.025 


4.075 


4.075 


245.025 



Table XXVIII. — Linkage Relations — Crossing-over only in the Sex 

HOMOZYGOTE, NoN-SEX-LINKED FACTORS 



Pi 


Percentage of 
crossing-over 


Gametic ratio 


Phenotypic ratio per thousand 


AB:Ah:aB:ah 


AB 


Ah 


aB 


ah 


Ah X aB 


n 


100 -71.100 -n_^ 
n n 


500 


250 


250 







1 


99.0:1:1:99.0 


747.5 


2.5 


2.5 


247.5 




2 


49.0:1:1:49.0 


745.0 


5.0 


5.0 


245.0 




3 


32.3:1:1:32.3 


742.5 


7.5 


7.5 


242.5 




4 


24.0:1:1:24.0 


740.0 


10.0 


10.0 


240.0 


X 


5 


19.0:1:1:19.0 


737.5 


12.5 


12.5 


237.5 


pO 


6 


15.7:1:1:15.7 


735.0 


15.0 


15.0 


235.0 


Q 


7 


13.3:1:1:13.3 


732.5 


17.5 


17.5 


232.5 





8 


11.5:1:1:11.5 


730.0 


20.0 


20.0 


230.0 


e 


9 


10.1:1:1:10.1 


727.5 


22.5 


22.5 


227.5 


X 


10 


9.0:1:1: 9.0 


725.0 


25.0 


25.0 


225.0 


cq 


20 


4.0:1:1: 4.0 


700.0 


50.0 


50.0 


200.0 


30 


2.3:1:1: 2.3 


675.0 


75.0 


75.0 


175.0 




40 


1.5:1:1: 1.5 


650.0 


100.0 


100.0 


150.0 




50 


1 :1:1: 1 


625.0 


125.0 


125.0 


125.0 



CHAPTER VII 
THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 

In previous chapters the formal relations which exist in the trans- 
mission of factors from parent to offspring have been discussed. It has 
been shown that these relations may be ascribed to the locus positions 
which factors occupy in the chromosomes. This single assumption 
taken together with the known behavior of the chromosome mechanism 
in its cycles explains very simply the two great categories of inheritance 
with respect to distribution of factors, namely independent segregation 
and linkage. Obviously, however, these are merely formal considerations, 
it is of considerable importance to know something about the factors 
themselves and the physiological interactions which they display with 
one another in the development of characters in the individual. It is 
to this problem that this and following chapters are addressed. 

It is true that as yet we know next to nothing about the factors 
themselves with respect to their physical and chemical constitution, we 
know them merely by their actions. We regard them as the loci ar- 
ranged in a linear series in the chromosome, we know they have certain 
characteristic effects in development and by these effects we recognize 
them. It is important to note that our knowledge of their behavior 
even is based on factor differences, not op a study of the factors them- 
selves. Thus we know that a certain locus in the germinal substance 
in Drosophila is concerned with the production of red eye color because 
when it is changed in a particular fashion, the eye color developed is 
no longer red, but white. We have no means of knowing how profound 
the relation of this factor to the other factors in the system is, nor can 
we judge as to the nature of the change in the locus by which the course 
of development was shifted from red to white in the production of eye 
color in Drosophila. Nevertheless a few things at least are known con- 
cerning the effects of factors in development and even in this vague field 
more and more facts are being discovered all the time. 

Factors are the genetic representatives of certain characters. Thus if 
a fly has a genetic constitution containing, among other factors for eye 
color, the factor w, then it will develop white eyes. In this particular 
case the eye color is practically the only character affected. Similarly 
in corn, if a mutation occurs in one of the basic aleurone color factors, 
for example, a change in the chromogen factor C to c then that corn thus 
9 129 



ISO GENETICS IN RELATION TO AGRICULTURE 

developing is white as respects aleurone color. Here apparently only 
aleurone color is concerned. Similarly in other cases much more insig- 
nificant changes may be connected with definite factor differences. Thus 
a forked condition of the spines in Drosophila is dependent upon a 
definite factor difference, a recessive factor in this case. One could 
go on and recount indefinitely factors which cause only very slight 
character changes. Any character change, therefore, however slight, 
may be based on genetic factor differences. The only valid genetic 
test is the pedigree breeding method, at the same time giving due con- 
sideration to environmental influences which may obscure or temporarily 
cover entirely the underlying genetic differences. 

Very great somatic differences may also be dependent upon differences 
in single factors in individuals. Perhaps the most striking of these are 
large size differences such as are found in beans, peas, and even in animals 
at times. Thus in beans the main difference between pole and bush 
beans is dependent upon a single factor difference. The difference 
between tall and dwarf varieties of peas is of a similar nature and has 
been fully discussed above. Certain types of dwarfing in man appear 
to depend upon single factor differences and in Drosophila there are 
factors which determine the production of giant races and others of 
dwarf races. Moreover, factor differences show striking relations to 
one another. Thus in Drosophila there are factors for eye color which 
change the shade of red in the eye, some resulting in a darker and many 
in lighter shades, but there is also a single factor difference which results 
in white eyes or in other words in the entire loss of color in the eyes, 
and even further there is a factor for an eyeless condition, which when a 
part of the genetic constitution of a fly results in the production of mere 
rudiments of eyes or even none at all. 

Very frequently single factors may cause such profound changes as 
to alter the entire appearance of the individual and interfere more or 
less with all its functions. Such, for example, is the case with fasciated 
forms in plants, some of which at least are dependent upon simple factor 
differences. A striking case of this type has been reported by O. E. White 
in tobacco. In this fasciated variety the number of leaves is greatly 
increased, from 24 to as high as 80, the stem is flattened and exhibits 
a characteristic fasciated condition, and the flowers are very abnormal. 
The abnormality of the flowers extends to every part, the numbers of 
sepals, petals, stamens, and ovary locules are increased, and striking 
deformities of these parts give evidence of the disturbing effect of the 
factor. The abnormal effects of the factor are not confined to external 
characters, but cytological studies show that the division figures, par- 
ticularly in reduction, show marked irregularities which may be expressed 
in an increase in the number of chromosomes, or in a breaking down of 



THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 131 

cells during division, or in various other peculiar phenomena. The ab- 
normal variety also displays a certain degree of sterility, probably asso- 
ciated with abnormal cell division. In spite of all the differences both 
external and internal which this mutation displays when compared with 
the normal variety from which it arose, its behavior in inheritance shows 
clearly that only a single factor difference is involved. When crossed with 
the normal type, the Fi is intermediate, and in F2 segregation is in 
approximately the ratio 1 abnormal: 2 intermediate:! normal. The F2 
homozygous segregants are exact duplications of the original pure forms, 
the normal segregants are in every respect as normal as the normal parent 
and the abnormal segregants are no less abnormal than those of pure 
abnormal races. The heterozygous forms are throughout clearly dis- 
tinguishable from abnormal homozygotes on the one hand and normal 
homozygotes on the other. Taken as a whole it would be difficult to find 
a better example of the profound effects which may result from a single 
factor difference. 

Lethal factors also exist which affect vital organs and result in the 
death of individuals homozygous for them. Excellent examples of such 
disturbing factors are those which affect the production of chlorophyll 
in plants. A number of species of plants at times produce races in 
which under experimental conditions approximately one-fourth of the 
seedlings are yellow or white instead of green and hence die soon after 
germination. Svich strains are particularly common in cereals, and in 
maize in almost any variety when a large number of self-fertilized ears 
are tested, a number of strains may be found which produce seedlings 
about one-fourth of which die as soon as the food supply of the endosperm 
is exhausted on account of deficiency in chlorophyll production. 

Since the homozygous recessive forms of albino strains die soon after 
germination, it follows that such strains must be propagated by means 
of the heterozygous individuals. The operation of such a scheme is 
illustrated in the following case. The original self-fertilized ear gave 
on germination 3 fully green seedlings to one which was pure white and 
which died shortly after germination. If we call the albino factor g 
in this case and its normal allelomorph present in the green plants G, we 
may assume that this ear was produced by a heterozygous green plant 
of the constitution Gg. Half the pollen grains of such a plant carry the 
factor G and half the factor g; and likewise in the ovules half bear the 
factor G, and half g. By self-pollination of such a plant, random fertili- 
zation of the ovules by the pollen grains results in grains in the ratio 
lGG:2Gg:lgg. Although grains of these different genotypes are indis- 
tinguishable in appearance, those of the genetic constitution GG and Gg 
produce fully green plants, while those which are gg produce albino 
seedlings which are incapable of independent existence on account 



132 



GENETICS IN RELATION TO AGRICULTURE 



of their lack of chlorophyll. Those green plants of the genetic constitu- 
tion Gg when self-fertilized produce grains one-fourth of which again 
give albino seedlings as in the previous generation. The green plants 
of the genetic constitution GG, however, since they are homozygous for 
the factor G produce nothing but green plants in succeeding generations. 



R 




« m « 08 



@H @ (D t 




Fig. 58. — Disturbance of phenotypic ratio by a recessive sex-linked lethal factor. Com- 
pare with Fig. 36. 

Morgan has demonstrated the existence of a number of lethal factors 
in DrosophUa. These factors result in the death of the individuals at 
some time before they reach the adult stage. They are particularly 
found among sex-linked factors, because sex-linked recessive factors 
have no normal allelomorphs in the male. The results of the presence 
of sex-linked lethal factors is shown diagrammatically in Fig. 58. As 
in corn, the strains are propagated by means of heterozygous individuals. 
Since such individuals can only be females in the case of sex-linked 



THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 133 

factors in Drosophila, it follows that the strains must be propagated by 
means of females heterozygous for the factors. The diagram shows 
how such lines are maintained. A heterozygous female produces eggs 
half of which bear the normal factor L, and half bear the lethal factor /. 
When mated to a normal male the X-chromosome of which bears the 
normal factor L, half the daughters are normal homozj^gotes and half 
are heterozygous for /. Half the males receive an A'-chromosome bear- 
ing the factor L, and consequently are normal and half receive an X- 
chromosome bearing the factor /. These latter die before reaching the 
adult stage, consequently a heterozygous female produces flies two- 
thirds of which are females and one-third males. The unusual sex- 
ratio provides a convenient test for heterozygous females and by this 
means the strain may be continued. 

Some of the consequences of the presence of lethal factors when 
linked with other factors are of importance because of the disturbances 
to which they give rise in Mendelian experiments. An illustration of 
such effects may be taken from Lethal III in Drosophila which is located 
at about the locus 26.5 in the X-chromosome. It is about 25 units 
distance from the locus for white eyes. If now a white-eyed female 
heterozygous for Lethal III be crossed with a red-eyed male, as shown 
diagrammatically in Fig. 58 all the females will be red-eyed but only 
half will be homozygous for the normal factor L3. These females, 
homozygous for L3, produce flies in the ratio of 1 red 9 : 1 red cf : 1 white 
9 : 1 white d^ when mated to their brothers. The other half of the Fi 
females, on the other hand, will be heterozygous for L3 and conse- 
quently, since crossing-over takes place in 25 per cent, of cases, they 
produce gametes in the ratio S{wkX):3(WL3X):l{Wl3X):l(wL3X). 
When such a female is mated to an Fi male fly the ratio is distinctly 
different from that obtained with the other females, in this case 4 Red 
9 :3 Red d^:4 White 9 :1 White cf. The ratio of sexes in this latter 
case is 2 female: 1 male and the same is true in Fi. The sex ratio gives 
an immediate clue to the disturbing factor and leads to a true explanation 
of the cause of the disturbance. 

Manifold Effects of Factors. — In a preceding section of this chapter it 
has been shown how far reaching may be the effects of single Mendelian 
factors, and in the present account it is intended to deal specifically 
with what Morgan has termed the manifold effects of single factors. 
Careful study has revealed the fact that although factors are restricted 
in their conspicuous results to certain characters, nevertheless they 
may have other less noticeable results which are none the less definite 
and constant. Baur has observed for example in Antirrhinum that 
the factor which produces pure white blossoms also yields plants which 
are distinctly weaker in growth and are smaller than those which possess 



134 GENETICS IN RELATION TO AGRICULTURE 

the normal allelomorph for this factor. Plants possessing the recessive 
factor may be recognized in the seedling stages by a peculiar coloration 
of the edges of the leaves and even better by the characteristic epidermis 
of the leaf blades. 

Manifold effects of factors are probably very common but very little 
definite work has been reported along this line. Morgan, however, has 
called attention to some cases in Drosophila. Thus there is a factor for 
club wings, and in strains of this type flies appear the wing pads of which 
fail to unfold after emergence. But this character is not constant, in 
fact about 80 per cent, of the flies in a pure strain have normal wings. 
Subsequent study, however, has shown that in such stocks the absence 
of spines on the side of the thorax is a constant differential test. These 
differences are shown in the accompanying figure (59). By employing the 
absence of spines as the differential test it is possible to classify mixed 
populations of "normal" and "club" flies accurately without paying any 
attention to wing characters. 

The Variability of Factor Expressions. — Factors also vary in the effects 
which they produce. We have pointed ovit that in pure strains of club- 
winged Drosophila (Fig. 59) only about 20 per cent, of the flies exhibit the 
unfolded wing pad characteristic of tlie club mutation. On the other 
hand, the absence of spines on the side of the thorax determined by the 
same factor appears to be an invariable characteristic of the club-winged 
flies. 

Sometimes this variability in factor expression may be traced to a defi- 
nite environmental condition. This is certainly true of the red Primula 
which produces red flowers under ordinary temperature conditions, 
but which when placed under abnormally high temperatures produces 
white flowers. The production of chlorophyll in some strains of corn, 
likewise, depends on generally favorable environmental conditions. 
This has been demonstrated by Miles for the yellow-green type of 
chlorophyll reduction. Plants heterozygous for this factor produce 
grains three-fourths of which produce fully green plants on germina- 
tion, but the other one-fourth produce pale yellowish seedlings with a 
tinge of green. The yellowish seedlings die under ordinary conditions, 
but in particularly favorable surroundings they continue to live and soon 
develop the normal chlorophyll coloration. If self -fertilized, they produce 
only yellowish plants which must again be given very favorable condi- 
tions for the production of the normal green leaf color. 

In Drosophila a number of environmental relations have been de- 
scribed. Thus Morgan has studied in considerable detail the influence 
of environment on the development of abnormal abdomen. Flies with 
the dominant factor for abnormal abdomen should all exhibit the char- 
acteristic type of deformed abdomen shown in Fig. 60; but this is not the 



THE NATURE AND EXPRESSION OF MEN DELI AN FACTORS 135 

case, for pure mutant stocks constantly show a high percentage of flies 
with normal abdomens. This variability in abdomen characters has 





Fig. 59. — Club-winged Drosophila. At a characteristic unfolded wing pads. At c 
the absence of spines on the side of the thorax is shown in comparison with the normal con- 
ditions, b. {From Morgan.) 




Fig. 60. Mutant type of Drosophila ampelophila called abnormal abdomen (the 
wings have been cut off); a, female; b, male; c, female that approaches the normal type. 
Development of this character is dependent upon moisture. (From Morgan.) 

been found to depend upon the condition of the food. When the food 
is moist a high percentage of flies have abnormal abdomens, but when the 
larvae are raised on dry food nearly all of them have normal abdomens. 



136 GENETICS IN RELATION TO AGRICULTURE 

On account of these relations the expected Mendehan behavior of 
this factor in crosses with normal flies is obscured in cultures grown 
on dry food, but with moist food Mendelian expectations are completely 
fulfilled. 

Moreover, the variability in the expression of the abnormal condition 
of the abdomen is not connected with any variability in the factor itself 
but is merely an expression of a variable reaction of the factor to the 
environment. Normal flies possessing the factor for abnormal abdomen 
when given moist food produce offspring just as abnormal as those from 
abnormal flies. The factor itself is invariable just as in a chemical 
system the elements which are in the system are invariable but may 
produce different results according to the dilution, temperature, and other 
conditions under which the reaction is going on. 

The reduplicated stock in Drosophila shows similar relations to en- 
vironmental conditions. The characteristic feature of this mutation is 
the production of extra legs or parts of legs. At normal temperatures 
very few flies show this condition, but when strains are grown at 10°C. 
a high percentage of them show supernumerary legs. As with ab- 
normal abdomen and moist food, so Miss lloge has shown that with 
temperatures below 10° these flies satisfy Mendelian expectations when 
crossed with normal strains, but at ordinary temperatures of cultivation 
the phenomena are entirely obscured. 

Duplicate Factors. — A number of cases are known where similar or 
identical effects are produced by factors located in different loci in the 
germinal substance. A case in point which has been subjected to excel- 
lent analysis is that for capsule form in the common shepherd's purse 
(Bursa) . When the form having flattened triangular capsules is crossed 
with that having top-shaped seed pods, the Fi plants produce triangular 
capsules. When the F2 is grown approximately 15 produce triangular 
capsules to one which produces top-shaped capsules. 

Such a result may be explained by assuming that two recessive factors, 
c and d, combine to produce the top-shaped capsule. The top-shaped 
race then is of the genetic constitution ccdd, and the contrasted tri- 
angular-shaped race is CCDD. The factors C and D are fully dominant 
and produce identical results, namely plants bearing the typical tri- 
angular-shaped seed pods. Consequently selfing Fi plants of the genetic 
constitution CcDd gives F2, 15 plants with triangular pods to 1 with 
top-shaped pods. The checkerboard for this case is shown in Fig. 61. 

If this analysis is valid for the inheritance of capsule form the F3 
and subsequent generations should display a characteristic type of 
behavior as shown in the checkerboard. In each square is given the 
ratio in which the particular genotype should segregate in F3. Thus it 
will be seen that 



THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 137 



7 families should breed true for triangular capsules. 

4 families should give 15 triangular :1 top-shaped. 

4 families should give 3 triangular: 1 top-shaped. 

1 family should breed true for top-shaped capsules. 
Shull applied this test to his cultures and obtained substantial agreement 
with theory throughout. Fig. 62 gives a graphic summary of his experi- 
mental results. 



c/— CD 



Cd 

Y 



cD 



f 



CD- 



Cd- 



cD- 



cd- 



CD. CD 



1:0 



Cd-CD 



1:0 



cD-CD 



1:0 



cd CD 



CD-Cd 



1:0 



Cd-Cd 



1:0 



cD-Cd 



15:1 



cd- Cd 



3:1 



CD. cD 



1:0 



Cd-cD 



15:1 



cDcD 



1:0 



cd^cD 



3:1 



CD. cd 



15:1 



Cd-cd 



3:1 



cD • cd 



3:1 



cd ■ cd 



0:1 



Fig. 61. — Checkerboard diagram to visualize the genetic relations in a dihybrid Ft 
family of Bursa bursa-pastoris X Heegeri, in respect to the capsule-characters. The capsules 
figured in each square indicate by their outline their phenotype, and by their oblique 
ruling their genotype, the gene C being represented by lines from upper right to lower left, 
and D from upper left to lower right. Homozygotes are densely lined, heterozygotes more 
sparsely. The ratios indicate the expectation in Ft when a plant having the genotypic 
constitution indicated in the same square, is self-fertilized. {After Shull.) 

When three duplicate factors are concerned in a hybrid the ratio in 
7^2 is 63 : 1, with four factors, 255 : 1, and so on. The first case of dupHcate 
factors was that described by Nilsson-Ehle in wheat. Here the red 
color of certain races of wheat depends on the presence of three dominant 
Mendelian factors so that such races are to be represented by the genetic 
formula RRSSTT and the contrasted white race by rrsstt. The Fi 
of a cross between two such races is of a pale red color intermediate 
between the parental red and white, and in F2 all shades of red are 
found from very pale to about the same depth of color as the parent 



138 



GENETICS IN RELATION TO AGRICULTURE 



race. In the actual experiment among seven families comprising a total 
of 440 plants only one produced white grains, but the F3 generation 
demonstrated the adequacy of the three-factor analysis. The inter- 







: I 1 I 


tit t 




it":? 


- J- s 


t § 


J- I S 


1 I 1 


' I 1 




^ T >^ 


I I 5^ 


t 4 § 




t 1 S 


4- t S 




t t § 


t 1 s 


ill 


J- 4 § 


ill 


i s 


1 X 




^ 1 ^ 






4 -L 


1 ^ 


1 -. '- L 1 


i. J S 


^ ^,^ I S 


1. s 


■ --^ t^ 1 




t s+l i 


/ t_ .- 1^1^ 1 


{)<' wfwv § 


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- ' ^ T~- ■ " r ■ '^ ~ u- ^ 


n ^~ •'— 7 r •• ; .1 ^ 


; V ;i , 




F C ! , Hi \ 11 '/ 


-^ rr -' r \ T" -^\ i - 




y '.\ . ':^ \ JA J i ' 


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It 1 ! ; ^ t T ' ■ "1 


v i \ --.-• ■ :Jr-] 


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>-'T / 1 ■ ' 


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I ,^' |! \/ ' * 




^_j^N. ^ 1 — ^ 


"?" 1 'in ' 


-F^; ^< N^^^^ ':;}: ! 1 i ■ ' 




4^v 1 ',T y' 


1 ^,. f ' \.y( y 


1 ^v \ < y 


^v 1 EI / 


■^ ^/r' 




1 1 1 1 1 . 1 1^ ■ 


jp t _ ■ ■ UTLT 


Fo t ' .'■K ■'' ~ 


1 ', >L 


1 I ^^ 


I i N 


t IN 


i 4 I^i 


J- 4 ^>. 




lis 


^ t 4 s 




.076-68.528 

250-73.860 

.340-73.096 

.028 - 83.036 

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.616-90.228 

.716-93.030 

.320-95.436 

.428-97.396 

.140-93.760 

.256-99.628 

.876 - 100.00 
100.090 



Fig. 62. — Resume of ratios found in 142 families in the first five generations following 
the cross;between Bursa bursa-pastoris and B. Heegeri. Each square represents a possible 
family, the position of a family being determined by the percentage of plants with tri- 
angular capsules as indicated at the base of the figure. {After Shull.) 

mediate shade" of red produced in Fi and the varying shades produced in 
segregation depend on the cumulative effect of the color factors. In- 



THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 139 

stead of displaying complete dominance for any one member of the 
factor system as Shiill found for the triangular capsule factors in Bursa, 
the factors here have a certain effect in color production which is additive, 





RrSsTt 






RrSsTt 






RrSsTt 






RrSsTt 








RrSsTt 








rrSsTt 


RrSsTt 


RrSSTt 






rrSsTt 


RrSsTt 


RrSSTt 






rrSsTt 


RrSsTt 


RrSSTt 






rrSsTt 


rrSsTT 


RrSSTt 






RrssTt 


rrSsTT 


RrSsTT 






RrssTt 


rrSSTt 


RrSsTT 






RrssTt 


rrSSTt 


RrSsTT 






RrssTt 


RRssTt 


RrSsTT 








RrSstt 


RRssTt 


RRSsTt 








rrssTt 


RrSstt 


RrSStt 


RRSsTt 


RrSSTT 






rrssTt 


RrSstt 


RrSStt 


RRSsTt 


RrSSTT 






rrSstt 


RrSstt 


RrssTT 


RRSsTt 


RRSsTT 






rrSstt 


rrssTT 


RrssTT 


rrSSTT 


RRSsTT 






Rrsstt 


rrSStt 


RRSstt 


RRssTT 


RRSSTt 




rrsslt 


Rrsstt 


RRsstt 


RRSstt 


RRSStt 


RRSSTt 


RRSSTT 



Fig. G3.—-F2 squares of the checkerboard of a cross of red (RRSSTT) X white (rrsstt) 
wheat arranged in classes according to the depth of color displayed by the phenotypes. 



i.e., two factors produce twice the depth of red coloration in the grain 
that one produces and all six are necessary for the production of the 



140 GENETICS IN RELATION TO AGRICULTURE 

full color of the parent red wheat. Consequently there are six shades 
of red in an F^ population possessing various frequencies with respect 
to the proportionate number of individuals which display a particular 
shade of color as shown in the foregoing diagram (Fig. 63). Factors 
which display summation effects have been conveniently called cumu- 
lative factors. 

Besides dominant factors which produce similar or identical somatic 
effects a large number of recessive factors are known which display the 
same phenomena. The first example of this type which was worked 
out was that in sweet peas described by Bateson. In sweet peas there 
are a number of different whites which phenotypically cannot be distin- 
guished from one another. The fact that they are genetically different 
is shown when they are crossed together, for then instead of producing 
white sweet peas the Fi plants bear colored flowers, the particular color 
depending upon the genetic constitutions of the whites which were 
crossed. Since the simultaneous action of two dominant factors, neither 
one of which by itself can produce any color, is necessary for color pro- 
duction, Bateson has proposed to call such factors complementary 
factors. 

The same relations have been found to exist in the production of 
aleurone color in grains of corn. Certain white varieties of corn are 
known which when crossed together give red or purple corn according 
to the genetic constitutions of the races which were crossed. As with 
dominant duplicate factors this sort of phenomenon gives peculiar 
Mendelian ratios in F^ because of the fact that many of the genotypes 
are indistinguishable phenotypically. Thus for example we may repre- 
sent a purple corn by the formula CCPP, these factors being particularly 
concerned in the production of aleurone color. A mutation in the 
locus C would give a white corn of the genetic constitution ccPP, and 
likewise a mutation in the locus P would give a white corn of the genetic 
constitution CCpp. Phenotypically these two varieties of white corn 
are indistinguishable, but from a genotypic standpoint the factors for 
white are located in different chromosomes in the two varieties. Accord- 
ingly when two such white varieties are crossed, the Fi is of the genetic 
constitution CcPp. Since C and P are both completely dominant over 
their allelomorphs c and p such a corn will be purple because the com- 
plete set of factors necessary for the production of purple aleurone color 
has been brought together by crossing these two genetically different 
whites. 

The checkerboard for the F2 of such a cross is shown in Fig. 64. It 
will be observed that the phenotypic ratio in F2 is 9 purple:? white. 
This is merely a modification of the typical 9:3:3:1 F2 ratio, for in this 
cross the last three classes are phenotypically alike, although geno- 



THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 141 

typically different. Of the nine purples, only one breeds true in Fz, 
and of the remaining eight purples, four give families which segregate 
in the ratio of 3 purple : 1 white, and four give families showing segrega- 
tion in the ratio of 9 purple: 7 white. All the whites, although of dijfferent 
genotypes, produce entirely white families. All these relationships are 
shown clearly in the checkerboard. 

In Drosophila a large number of similar cases of like somatic effect 
have been found to be dependent upon different factors. Here the 
linkage values of the different factors with other factors have been 
determined very precisely, and moreover the mutants have for the most 
part arisen directly from the cultures, so that the relationships have 
been established much better than in any other form. 



CP 



Cp 



cP 



cp 



CCPP 

Purple 

1:0 


CCPp 

Purple 

3:1 


CcPP 

Purple 

3:1 


CcPp 

Purple 

9:7 


CCPp 

Purple 

3:1 


CCpp 

White 

0:1 


CcPp 

Purple 

9:7 


Ccpp 

White 

0:1 


CcPP 

Purple 

3:1 


CcPp 

Purple 

9:7 


ccPP 

White 

0:1 


ccPp 

White 

0:1 


CcPp 

Purple 

9:7 


Ccpp 

White 

0:1 


ccPp 

White 

0:1 


ccpp 

White 

0:1 



CP 



Cp 



cP 



cp 



Fig. 64. — Checkerboard of F2 of cross white (ccPP) X white (CCpp) maize, showing 
phenotypes and F3 segregation as well as genotypes. 



For body color at least three similar mutant factors result in almost 
identical darker forms. The first of these to be discovered was the black 
factor which is located in the second group of factors. The factor for 
ebony body color is in the third group, and sable is a sex-linked factor. 
Although so nearly alike that a mixed population could not be certainly 
classified these particular races do show slight differences in coloration. 
Similarly nearly identical results are obtained from three different 
jaunty factors which cause the wings to turn up at the ends. Morgan 
has also pointed out other such similarities in effect of different factors 
which affect eye and wing characters, color, etc. 

Sometimes a dominant and a recessive factor give identical pheno- 
typic results. For an illustration of this we may again turn to aleurone 



142 



GENETICS IN RELATION TO AGRICULTURE 



color in corn. Taking into account the white dominant factor for 
aleurone coloration, the following genotypes may be obtained: 

WWccPP = white 
wwCCPP = purple 

wwccPP = white 

wwCCpp = white 
wwccpp = white 



WCP 



WCp 



WcP 



Wcp 



wCP 



wCp 



U'CP 



WCP 



WCp 



WcP 



Wcp 



wCP 



wCp 



wcP 



wcP 



WWCCPP 

White 

0:1 


WWCCPp 

White 

0:1 


WWCcPP 

White 

0:1 


WWCcPp 

White 

0:1 


WwCCPP 

White 

1:3 


WwCCPp 
White 
3:13 


WwCcPP 
White 
3:13 


WwCcPp 
White 
9:55 


WWCCPp 

White 

0:1 


WWCCpp 

White 

0:1 


WWCcPp 

White 

0:1 


WWCcpp 

White 

0:1 


WwCCPp 
White 
3:13 


WwCCpp 

White 

0:1 


WwCcPp 
White 
9:55 


WwCcpp 

White 

0:1 


WWCcPP WWCcPp 

White White 

0:1 0:1 


WWccPP WWccPp 

White White 

0:1 0:1 


WwCcPP 
White 
3:13 


WwCcPp 
White 
9:55 


WwccPP 

White 

0:1 


WwccPp 

White 

0:1 


WWCcPp ' WWCcpp 

White ! White 

0:1 1 0:1 


WWccPp 

White 

0:1 


WWccpp : WwCcPp 

White \ White 

0:1 9:55 


WwCcpp 

White 

0:1 


WwccPp 

White 

0:1 


Wwccpp 

White 

0:1 


WwCCPP 

White 

1:3 


WwCCPp 
White 
3:13 


WwCcPP 
White 
3:13 


WwCcPp 
White 
9:55 


wwCCPP 

Purple 

1:0 


wwCCPp 

Purple 

3:1 


wwCcPP 

Purple 

3:1 


wwCcPp 

Purple 

9:7 


WwCCPp 
White 
3:13 


WwCCpp 

White 

0:1 


WwCcPp 
White 
9:55 


WwCcpp 

White 

0:1 


wwCCPp 

Purple 

3:1 


wwCCpp 

White 

0:1 


wwCcPp 

Purple 

9:7 


wwCcpp 

White 

0:1 


WwCcPP 
White 
3:13 


WwCcPp 
White 
9:55 


WwccPP 

White 

0:1 


WwccPp 

White 

0:1 


wwCcPP 

Purple 

3:1. 


wwCcPp 

Purple 

9:7 


wwccPP 

White 

0:1 


wwccPp 

White 

0:1 


WwCcPp 
White 
9:55 


WwCcpp 

White 

0:1 


WxvccPp 

White 

0:1 


Wwccpp 

White 

0:1 


wwCcPp 

Purple 

9:7 


wwCcpp 

White 

0:1 


wwccPp 

White 

0:1 


wwccpp 

White 

0:1 



Fig. 65.- 



-Fi checkerboard for cross of white (WWCCPP) X white (wwccpp) corn. 
In the Fs segregation ratio the purple is given first as in Fig. 64. 



The student will be able to figure out many different relations which 
exist when such races are crossed. In this section only one will be con- 
sidered as an illustration of the working of such a sj^stem. If a white 
corn, WWCCPP, is crossed with a white corn, wwccpp, the Fi is of the 
genetic constitution, WwCcPp, and is white on account of the action of 
W. The F2, however, shows some purple grains as will become apparent 
from a study of the accompanying checkerboard, Fig. 65. In F2 such a 
hybrid segregates in the ratio 55 white : 9 purple, and in F^ the families 
show the segregation ratios indicated in the proper squares of the checker- 



THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 143 

board. As in the previous instance these ratios are merely modi- 
fications of the typical Mendelian dihybrid and trihybrid ratios due to 
the fact that many of the classes are white and hence are merged into 
one. 

It should be apparent from the discussion in this chapter that many 
complex relations exist as respects the nature and expression of factors. 
Only some of the best established and most conspicuous cases have been 
discussed and some of these in rather incomplete fashion, but the material 
presented is sufficient to establish several facts concerning factors, 
namely that some factors have very minute, others very far reaching 
effects, that factors may affect many characters in the individual, that 
factors may vary in their expression in individuals, that sometimes this 
variability in factor expression is dependent upon definite environmental 
conditions and sometimes on obscure or unknown causes, and that at 
times different factors may have similar somatic expressions. It is 
difficult to treat such various matters in any systematic fashion, con- 
sequently this chapter must be regarded merely as an introduction to 
the general topic of factor interactions. 



CHAPTER VIII 
ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 

The present chapter is designed to deal with those relationships in 
which a single locus in the hereditary system is involved. 

Mendel worked with seven pairs of contrasted characters and he ob- 
served that in all of these one member of the pair controlled the expres- 
sion of the character when the individual was heterozygous. When tall 
peas are crossed with dwarf the hybrid is tall, in fact slightly taller even 
than the tall parent. Similarly yellow cotyledons are dominant over 
green, and smooth over wrinkled seed. The same is true for the other 
four pairs of characters. So important did this fact of dominance appear 
to investigators that for some time after the rediscovery of Mendelism 
reference was very- generally made to the law of dominance, and great 
significance was attached to any failure to observe dominance in genetic 
investigations. But subsequent investigations have shown that domi- 
nance, far from being a general rule, is merely a special condition met 
with in certain cases of inheritance. That it is by no means universal 
must be conceded. How far it obtains and what other conditions are 
met with in its absence, we shall endeavor to show in what follows. 

Dominance is a relation existing between a factor and its allelo- 
morph such that in plants heterozygous for the factor in question the 
character expression is the same or approximately the same as that when 
the factor is homozygous. Dominance, therefore, applies only to rela- 
tions existing between a pair of factors. That two contrasted characters 
show an intermediate condition is no evidence in itself that dominance is 
lacking. It must further be demonstrated that this condition is due to 
the fact that the character expression of a genotype Aa lies between that 
oi AA and aa. Otherwise the intermediate expression of the hybrid 
character may be the expression merely of the action of several pairs of 
factors each displaying dominance for one member of each pair, but 
together giving an intermediate expression. 

The Extent of Dominance. — Off hand it would appear that com- 
plete dominance is a very common phenomenon in genetic investigations. 
The seven pairs of contrasted characters in peas could hardly have dis- 
played it in all the pairs unless it were a condition of wide occurrence 
and considerable significance. Otherwise we should have to consider 
this a remarkable case of coincidence. Likewise the oft-cited investi- 

144 



ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 



145 



gations with Drosophila 
indicate that usually a 
normal allelomorph is dom- 
inant to a mutant factor, 
and in facti often to the 
eye completely dominant. 
More precise investigations 
indicate, however, that al- 
though for all practical 
purposes dominance often 
is so complete as to closely 
approximate the expres- 
sion of the homozygous 
character due to the 
duplex ' condition of the 
dominant factor, still the 
completeness of dominance 
is often more apparent 
than real. 

Darbishire has attacked 
this problem in the case of 
the cross smooth as con- 
trasted with wrinkled peas. 
Mendel's experiments 
showed that smooth or 
round shape is dominant 
over the wrinkled shape in 
peas and as in other cases 
the dominance appears to 
the eye complete. Darbi- 
shire investigated the cause 
of the difference between 
round and wrinkled peas 
and found it associated 
with a difference in starch 
content. Thus during the 
development of the seed 
in those races possessing 
round seeds the sugar is 
almost wholly converted 
into starch so that when 
the seed is ripe and drying 
it retains water rather 




10 



146 GENETICS IN RELATION TO AGRICULTURE 

firmly and shrinks uniformly to form a round seed. Like the 
seeds of round races those of wrinkled peas are also round at the 
height of development, but in peas of such varieties the sugar is 
very incompletely transformed into starch. Consequently in ripening 
and drying they give up more water proportionally, than round 
races and do not shrink uniformly. As a result they become 
very much wrinkled at maturity. This difference in the starch grains 
of the wrinkled pea is not only a matter of less complete trans- 
formation of sugar into starch, but is also associated with less perfect 
production of starch grains as shown in Fig. 66. Thus in the round races 
the starch grains are numerous and are large and entire. They show 
practically no subdivision of the grains. But in the wrinkled peas the 
grains are not only less numerous, but they show fissures which give them 
an appearance like that of the compound starch grains of some species 
of plants. This appearance is probably due to the fact that actual 
breaking down of starch grains occurs in wrinkled peas during ripening 
so that the grains remaining are in a partial stage of disintegration. In 
the hybrid between a round and a wrinkled pea, however, the condition 
of the starch grains is intermediate between that of the two parents. 
The grains are intermediate not only in number and shape but also in 
the degree of disintegration they display. In the contrasted pair of 
characters, round vs. wrinkled seed in peas, the dominance of round is, 
therefore, merely a superficial character expression. Actually the basic 
phenomena involved, i.e., the transformation of sugar into starch, show 
an intermediate condition in the hybrid. The superficial character ex- 
pression of this intermediate condition happens to be the same as that 
of the strict parental round condition, so that dominance here is merely 
dependent on superficial resemblance. We may well hesitate, therefore, 
in our judgment as to the completeness of dominance in any case until 
it has been examined with considerable care. 

Sometimes the application of more precise character measurements 
will suffice to detect a difference between the homozygous and hetero- 
zygous character expression. This is shown for the case of miniature 
vs. long wings in Drosophila. In miniature- winged flies the wings reach 
about to the tip of the abdomen, whereas in the long-winged flies they 
extend considerably beyond the abdomen. The long-winged condition 
is dominant, to the eye completely, and there is absolutely no difficulty 
in segregating the long- winged flies of an F 2 population from those which 
have miniature wings. Nevertheless Lutz has shown that when biomet- 
rical methods are employed the length of wings of heterozygous flies com- 
pared with the length of legs is shorter than that for flies homozygous for 
the long-winged factor. The difference in character expression in this 
case is slight but it can be demonstrated by the employment of precise 
methods of measurement. 



ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 147 

Intermediate Expression in the Hybrid. — From those cases in which 
dominance is nearly or quite complete we may next pass to those in which 
the character expression of Aa is intermediate to that of AA and aa. 
There are numberless instances of this kind, and they are of interest 
because the heterozygous class may be distinguished in F2, so that the 
typical ratio obtained is lA:2Aa: la, instead of 3^ :1a as in cases where 
dominance occurs. 

For a concrete example we may turn to Baur's case in the snap- 
dragon. Baur and Miss Wheldale have independently conducted very 
extensive investigations of Mendelian inheritance in Antirrhinum. 
For most cases one member of a pair of contrasted characters is dominant, 
but when ivory is crossed with red the Fi is intermediate in color, it is 
pale red or pink. When F2 is grown it is found to consist of 1 red: 2 
pink: 1 ivory. In one case among 97 plants, Baur obtained 22 red, 

RR X rr 



Red 



Ivory 



Rr 
Pink 



1 RR 2 Rr 1 7T 

Red Pink Ivory 



RR 1 RR 2Rr 1 rr rr 

Red Red Pink Ivory Ivory 

Fig. 67. — Results of crossing snapdragons with red and ivory colored flowers. 

52 pink, and 23 ivory, a satisfactory agreement with Mendelian ex- 
pectations. The actual proof for this case comes out in growing F3. 
When this is done it is found that the red plants and the ivory plants 
give progeny which are entirely red and ivory, respectively. The pink 
plants on the other hand are all heterozygous and they give in 7^3 and 
in all succeeding generations plants in the proportion of 1 red: 2 pink:l 
ivory. The case is very evidently one in which a single factor difference 
is concerned. If the factor responsible for the production of red in 
Antirrhinum be designated by R, then we may designate its allelomorph 
present in the ivory race by r. The case then works out according to 
the diagram in Fig. 67. 

In the Four o'clock, Mirdbilis jalapa, it appears to be the rule that 
heterozygous plants present visible differences from plants homozygous 
for color factors. For this reason in breeding experiments this plant 
gives a rather remarkable diversity of colors with relatively few factors 
involved. Thus we may start with the primary assumption that in one 
series of colors we have involved two pairs of factors as follows: 

Y = factor for yellow colored sap. 

R = factor which turns yellow sap red. 



148 



GENETICS IN RELATION TO AGRICULTURE 



The various homozygous combinations of these two factors give four 
primary races which breed true as follows: 

YYRR = crimson. 

YYrr = yellow. 

yyRR = white. 

yyrr = white. 

By hybridizing these races four heterozygous forms may be produced 
which are of the colors given below: 

YYRR (crimson) X YYrr (yellow) gives YYRr = orange red. 
YYRR (crimson) X yyRR (white) gives YyRR = magenta. 
YYRR (crimson) X yyrr (white) gives YyRr = magenta-rose. 
YYrr (yellow) X yyrr (white) gives Yyrr = pale yellow. 



cT YR 


Yr 


yR 


yr 


9 
YR 


YYRR 
Crimson 


YYRr 
Orange red 


YyRR 
Magenta 


YyRr 
Magenta rose 


Yr 


YYRr 
Orange red 


YYrr 
Yellow 


YyRr 
Magenta rose 


Yyrr 
Pale yellow 


yR 


YyRR 
Magenta 


YyRr 
Magenta rose 


yyRR 
White 


yyRr 
White 


yr 


YyRr 
Magenta rose 


Yyrr 
Pale yellow 


yyRr 
White 


White 



. Fig. 68. — Checkerboard analysis of the progeny of a magenta-rose Mirabilis of the genetic 

constitution YyRr. 

We thus have seven distinct color classes as a result of various com- 
binations of two pairs of color factors. 

Moreover, this species gives a very good example of the diversity 
which may be obtained in an F^ population. Thus Miss Marryat 
has shown that when magenta-rose, YyRr, is selfed, the progeny fulfil 
the conditions indicated by the accompanying checkerboard analysis 
in Fig. 68. 



Table XXIX. — F2 Phenotypes and F3 Phenotypic Ratios Derived prom 
THE Original Cross, Crimson, YYRR X White, yyrr 


Color of parent 


Number of 
plants selfed 


Number of 
offspring 


Color of offspring 


Yellow 


2 
2 
3 
4 
3 
5 


26 
23 
61 
64 
46 
70 


All yellow. 
All crimson. 


Crimson 


Orange red 


17 crimson : 31 orange red : 15 white. 

18 crimson : 32 magenta : 14 white. 
9 yellow : 25 pale yellow : 12 white. 

5 crimson : 9 magenta : 6 orange red : 

19 magenta-rose : 3 yellow : 7 pale 
yellow : 21 white. 


Magenta 


Pale yellow 


Magenta-rose 



ALLELOMORPHIC RELATIONSHIPS IN MEN DELI SM 



149 



When the Fa was grown from such an F2 population Miss Marryat 
obtained excellent agreement with this analysis as is shown by the data 
in Table XXIX. 

Variable Character Expression in the Hybrid.^ — ^Sometimes the 
character expression in Fi while intermediate displays a range of varia- 
tion extending almost from one parent to the other. This is shown rather 
strikingly in the case of bar eyes in Drosophila (Fig. 69) . The bar eye 
factor is a sex-linked mutant factor which is responsible for the pro- 
duction of flies with long narrow eyes instead of the round eyes normal 
for the species. When a female with bar eyes is crossed to a normal 
male the F: all have bar eyes. In the males especially the eyes are 






Fig. 69. 



-Normal (a, o') and bar eye (b, b') of Drosophila; shown in side view and as seen 
from above. (After Morgan.) 



just as narrow as in homozygous races, but among the females some 
may be found which have eyes nearly as narrow as those characteristic 
of homozygous bar eye flies and others which have eyes nearly as round 
as those characteristic of the normal fly. Most of them, however, have 
eyes which display an intermediate effect of the factor. 

This case readily admits of explanation, if the genetic phenomena 
involved are considered. Since the factor for bar eyes is sex-linked we 
may represent the bar-eyed female as {B'X){B'X), following Morgan 
in employing the primed symbol to indicate a dominant mutant factor. 
The male with normal eyes is then (b'X) Y. When a bar-eyed female is 
mated to a normal male, bar-eyed females and males are obtained in 
Fi as shown in the diagram in Fig. 70. 

The Fi bar-eyed male obtains his only X-chromosome from the female 
and this chromosome contains the factor for bar eyes. He has exactly 



150 



GENETICS IN RELATION TO AGRICULTURE 



the same genetic constitution, therefore, as a male of a pure bar-eyed 
race, and it is to be expected that he will display the character to the 
same extent as a male from a pure race. On the other hand the female 
has one X-chromosome which bears the normal recessive allelomorph 
of the bar-eye factor. This factor may be considered as exerting a 
competitive influence against the bar-eye factor of the other X-chromo- 



Pi 



Gametes 



Bar-eyed 9 
{B'X){B'X) 



(B'X) 



X 




Normal cf' 
{b'X)Y 



(b'X) 



Fi (B'X) ih'X)-^ ^(5'Z) Y 

Bar-eyed 9 Bar-eyed cT 

Fig. 70. — Results of mating bar-eyed 9 with normal-eyed cf Drosophila. 

some, so that the character expression in a sense depends upon a variable 
equilibrium reached between the two factors. Since they appear to be 
nearly equal in potency it is possible apparently for this equilibrium to 
be thrown so much to one side or the other that at times the character 
expression approaches that of the typical bar-eyed strains and at times 
that of the normal round-eyed flies. 




Fig. 71. — Longitudinal sections of corn grains showing differences in character of starch; 

left, floury; right, flinty. 

An interesting case which throws considerable light on the competi- 
tive action of factors in determining character expression has been 
reported by Hayes and East in maize. Flint races of maize are char- 
acterized by the production of a very small amount of soft starch in 
the center of the seed and a large amount of hard corneous starch sur- 
rounding it. Flour corns on the other hand produce grains the endo- 
sperm of which is almost wholly made up of soft starch with occasionally 
a very thin layer of corneous starch at the exterior of the endosperm. 
These differences are shown in Fig. 71. 



ALLELOMORPHIC RELATIONSHIPS IN MEN DELI SM 151 

When a floury corn is pollinated by a flinty corn the grains which 
result show no effect of the flinty pollination, they are floury grains of 
the same character as those of a pure floury race. Similarly when a 
flinty corn is pollinated by a floury corn, the grains are flinty. Again 
they are of the same character as the maternal parent. The maternal type 
of grains is always produced in such reciprocal crosses. Following up 
this experiment, when Fi corneous grains of the cross corneous 9 X 
floury cf are grown and selfed, the ears produced show distinct segre- 
gation into flinty and floury corn in the ratio 1 flinty : 1 floury. Fi floury 
grains from floury 9 X flinty cf when grown and selfed likewise pro- 
duce ears showing distinct segregation into 1 flinty : 1 floury. Evidently 
the Fi grains although different phenotypically display the same genetic 
phenomena. 

Cytological research has shown that in the fertilization of maize 
and other plants there is a double fertilization, one fertihzation giving 
rise to the embryo and the other to the endosperm. In the case of the 
embryo, an egg nucleus unites with a nucleus from the pollen grain 
and from this fusion the embryo develops. In the fertilization which 
gives rise to the endosperm two nuclei from the female unite with one 
from the male, so that the cells of the endosperm contain 3a; chromo- 
somes rather than the duplex number characteristic of the cells of the 
embryo. If the flinty factor be represented by F, and the contrasted 
factor for floury by /, the zygote of a flinty corn is FF, but the endosperm 
connected with it is FFF. Correspondingly for the floury race the zygote 
is //, and its endosperm ///. In the fertilization of flinty by floury corn, 
the egg nucleus proper, the genetic constitution of which is F, is fertil- 
ized by an / pollen grain, giving a hybrid zygote of the constitution Fj. 
The endosperm which surrounds this embryo, however, arises from the 
fusion of the two endosperm nuclei, FF, with a single nucleus from the 
pollen grain, giving a zygote of the constitution FFf. This endosperm 
is flinty because two doses of F are apparently dominant to one dose of /. 
On the other hand, when floury corn is pollinated by flinty, the embryo 
has the same genetic constitution, namely FJ, but the endosperm sur- 
rounding it arose by union of two endosperm nuclei // with a pollen 
nucleus bearing the factor F. It, therefore, has the genetic constitution 
jJF and it is floury because the two doses of / determine the phenotypic 
expression to the exclusion of the single dose of F. In F2 the hybrid 
flinty grains from the cross flinty 9 X floury cT give exactly the same 
results as the hybrid floury grains from the cross floury 9 X flinty 6^. 
Here the ratio is 1 flinty: 1 floury in each case, and half the members 
of each class are heterozygous and will reproduce the same ratios in the 
succeeding generation. 

It would be difficult to conceive of a more beautiful illustration of 



152 GENETICS IN RELATION TO AGRICULTURE 

the quantitative relations obtaining in the determination of dominance. 
Apparently the relations are about the same as those shown in the ease 
of bar eye in Drosophila, for conceivably, if such a thing could be obtained, 
an endosperm arising from an Ff cell might show the same variation 
between flinty and floury that is shown in the bar-eye character of flies 
of the genetic constitution (B'X) (h'X). 

Mosaic Expression of the Hybrid Character. — ^Another type of hybrid 
condition is that in which the Aa individuals are a mosaic of the char- 
acters of the two parents. This condition is very strikingly illustrated 
in Blue Andalusian fowls. Andalusian fowls are of three types: 
black, splashed white, and the so-called blue. Of these types the black 
and splashed white breed true, but the blue is a hybrid and constantly 
segregates in the ratio 1 black : 2 blue : 1 splashed white. When 
black and splashed white are mated, the progeny are all blue. The 
Blue Andalusian fowl of the Poultry Standard of Perfection is, there- 
fore, a heterozygous form and for that reason all attempts to establish 
it as a pure breeding race have failed. The case, however, is of interest 
here because the Blue Andalusian is a peculiar mosaic of the characters 
exhibited by the black and splashed white. Its "blue" color is simply 
due to a fine but uneven sprinkling of black pigment through the 
feathers ; and on some portions as for instance the feathers of the breast, 
the black is present as a. distinct edging or lacing of the feathers. 

Similar mosiac hybrids which represent a simple heterozygous con- 
dition have been reported by Nabours in grouse locusts of the genus 
Parattetix. Nabours found nine distinct races which bred true for 
particular color patterns. Hybrids, however, between any two of these 
species display the entire color pattern of both parents, the color patterns 
being merely superimposed one upon another and in such a manner that 
the entire pigmentation of both parents is present in the hybrid and is 
distributed in the same fashion. If then two races of Parattetix A and 
B be crossed, the hybrid AB will be a mosiac of the two parents, and it is 
possible by simple inspection of such a hybrid form to determine what 
races entered into it. Such a hybrid will give a population consisting 
of 1A:2AB:1B, thus demonstrating that the case rests on a simple 
factor basis and that the mosaic pattern is simply an expression of a 
heterozygous condition in which both A and a, if we designate them thus, 
work out their full possibility in the development of the hybrid. In 
certain cases which did not appear to conform to this simple interpreta- 
tion, a microscopic examination was resorted to. This examination dem- 
onstrated that the lack of agreement was apparent rather than real. 
Thus in Fig. 72 the superficial characters of the hybrid (BI) between P. 
leuconotus (BB) and P. nigronotatus (II) are for the most part those of P. 
leuconotus except for the broad black band across the pronotum which is 



ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 



153 



clearly derived fromP. nigronotatus. In the posterior part of the pronotum 
particularly the characters of P. leuconotus, appear to be dominant but 
the microscopic study showed clearly that this was due to differences in 
distribution in the two parents, and that the characters of P. nigronotatus, 
although obscured were as much present as those of leuconotus. 








Fig. 72. — Three types of Paratettix, BB, CC, II, and two of the hybrids between them. 

(After Nabours.) 



The Presence and Absence Hypothesis. — The foregoing accounts of 
the relations existing in the expression of the hybrid characters as compared 
with the two parental characters serves as an adequate introduction for 
a brief consideration of the presence and absence hypothesis. Accord- 
ing to the presence and absence hypothesis as advanced by Bateson and 
Punnett, the only relations which can exist with respect to a certain fac- 
tor depend on its presence or absence from the hereditary material. 
Thus if we consider the factor R for round shape in peas, and its allelo- 
morph r for wrinkled shape, according to the presence and absence 
hypothesis the r of the genetic formula of the wrinkled pea is not itself a 
factor as we have assumed throughout the discussion in this text, but 
merely represents the absence of the factor R. The wrinkled character, 
therefore, is merely an expression of the action of the set of genetic fac- 
tors in peas when the factor R has been taken away from the system. 

In this text we have throughout assumed that the recessive symbols 
stand for factors just as truly as do the dominant ones, and we have 
regarded the difference between a recessive factor and its corresponding 
dominant allelomorph as dependent upon some change in a dominant 
factor sometimes profound and sometimes less profound so that all 



154 GENETICS IN RELATION TO AGRICULTURE 

variations from complete dominance to a strict intermediacy may be ob- 
tained among hybrids. For cases of complete dominance, the presence 
and absence idea satisJ&es conditions very satisfactorily as far as formal 
relations are concerned, and intermediacy and even other conditions 
of the hybrid expression may be assumed to depend upon the quantitative 
difference in the amount of the factor present in the hybrid race as 
contrasted with the parent races. Difficulties, however, begin to arise 
when attempts are made to explain the origin of dominant mutations in 
terms of this hypothesis, for in such cases it is almost necessary to assume 
that a factor has been added to the hereditary material. It is usually 
considered easy enough to account for a recessive mutation as due to the 
dropping out of a factor from the hereditary material, but when a factor 
is added to that material, we must ask from whence it came, what its 
nature, etc. If we regard mutations as simply due to changes in a fac- 
tor this difficulty vanishes for then dominance or recessiveness of the 
mutations depends merely on the relations between the mutated factor 
and its unchanged condition and there is no particular reason for as- 
suming that all mutations should be of the nature of "loss" mutations, 
i.e., mutations depending upon the loss of a factor from the hereditary 
material and resulting in the absence of some dominant character in 
the individuals concerned. There is no difficulty therefore, in account- 
ing for the four or five dominant mutations which have been observed in 
Drosophila, if we regard mutation as a change in a locus, for these par- 
ticular mutations simply happened to involve changes of such a type 
that the mutated locus was dominant to the unmutated condition. 
Obviously, also, such a view conforms more closely with the facts ob- 
served in cases of the competitive action of factors such as is seen in bar 
eyes in Drosophila or in the factors for flinty and floury endosperm in 
maize. 

But there are more serious objections than these which can be raised 
against the presence and absence hypothesis. In Drosophila, for in- 
stance, a number of cases of return mutations have been observed, 
many of them in cultures so controlled that the possibility of explaining 
them by chance contamination is practically precluded. Thus in stock 
so controlled by the presence of other factors that it would practically 
have been impossible to have a contamination go unnoticed on account 
of the introduction of other factors, the bar-eyed race of Drosophila has 
been known to produce normal-eyed mutants (May) and eosin-eyed flies 
have been observed to give white-eyed flies on several occasions; while 
on the other hand eosin, although dominant to white, originally arose as 
a mutant in a stock of white-eyed flies. If we assume that the change 
from eosin to white involves a relatively unessential change in the W 
factor in Drosophila, in chemical terms perhaps a slight rearrangement in 



ALLELOMORPH IC RELATIONSHIPS IN MEN DELI SM 155 

the. molecule or a change in an end radical, then it is not difficult to 
imagine how a reverse mutation might arise. Reverse mutations, there- 
fore, support the idea that the recessive member of an allelomorphic 
system is just as truly a factor as the dominant member. Never- 
theless these considerations do not in themselves confute the argument 
of presence and absence, although they tend to throw the weight of 
evidence strongly against it. It is, however, perhaps not amiss to point 
out that much of the weight of authority of the presence and absence 
hypothesis depends on the fact that it was advanced at the psycho- 
logical moment, and that, as Morgan points out, in the light of our 
present knowledge of the relation between factors and characters it 
assumes a knowledge far beyond that which we have at present attained. 
But the really serious objections to the hypothesis are those based on the 
evidence furnished by multiple allelomorphism. 

Since the foregoing was written Bridges has published results of his 
investigation of a case of loss or inactivation of a portion of the X-chro- 
mosome in Drosophila. The deficient section involved the factor for 
bar eye. As Bridges points out this constitutes the first valid evidence 
upon the question of presence and absence. According to the presence 
and absence hypothesis the original appearance of the dominant bar 
character was due to the loss from the chromosome of an inhibitor, 
thereby allowing the normal narrowing effect of the remaining complex 
to assert itself. It should make no difference whether this inhibitor 
were lost by a special loss involving only the inhibitor or whether it 
were lost because of being situated in a particular section which became 
lost. In other words, the chromosome which is deficient for the region 
carrying the inhibitor should allow the occurrence of the same narrowing 
effect that is allowed by the simple loss of the inhibitor. In point of 
fact, the deficiency of the region in which the inhibitor must be hypoth- 
ecated does not produce an effect like that of the mutation responsible 
for bar. For, the female carrying one deficient X and one normal X 
shows no narrowing of the eye shape, and likewise the female carrying 
one deficient A' and one bar X is no narrower in eye shape than a 
normal heterozygous bar. Thus, in the only case which has a direct 
bearing on the presence and absence hypothesis, it is seen that the ex- 
pedient of the loss of inhibitors to explain the origin of a dominant 
mutation is of no avail. 

Multiple Allelomorphism in General. — Multiple allelomorphism is 
the term applied to those cases which seem to depend on a series of 
changes in a given factor locus. Cuenot advanced such an explanation 
for the inheritance of certain color patterns in mice, and Morgan has 
since described several cases which occur in Drosophila. Since these 
later cases are simpler and have been worked out in more detail they will 
be treated first. 



156 GENETICS IN RELATION TO AGRICULTURE 

Multiple Allelomorphism in Drosophila. — A typical case is that 
centering around the locus for eye color in Drosophila which we have 
called W. This locus is situated in the X-chromosome at a distance of 
one unit from the locus Y for body color. The first mutations in Dro- 
sophila involved a change in W such that white eyes were produced, a 
mutation recessive to the normal red-eyed condition. This factor is 
called w and its inheritance has been dealt with in previous chapters. 
Later some flies arose in a white-eyed culture which had eosin eyes. 
When a white cf is mated to an eosin 9 the Fi is eosin ^ and F2 consists of 
3 eosin: 1 white. When a red-eyed 9 is mated to an eosin-eyed cf, 
Fi is red, and F^ segregates in the ratio 3 red:l eosin. The facts are 
explainable on the assumption that the factor W has been changed in a 
different fashion to produce the factor for eosin which we designated as 
w". On this basis the analysis of the genetic constitutions of these 
different races is as follows : 

{WX){WX) = red 9 {WX)Y = red & 

{iv''X){w'X) = eosin 9 {:ufX)Y = eosin cf 

{wX){wX) = white 9 {wX)Y = white d^. 

A change in the same locus has occurred in the mutation to white and 
to eosin, but the change has been different in each case. Later four 
other changes in this locus occurred giving eye colors which have been 
named cherry, tinged, blood and buff, and these fulfil the same conditions 
as those pointed out for eosin. The factors are designated w% w^, w^ 
and w'"^ respectively. These seven factors therefore display a particular 
type of behavior depending upon the fact that they occupy the same 
locus in the X-chromosome. They form together a system of septuple 
allelomorphs. 

In Drosophila there are at least three other such systems of multiple 
allelomorphs. One of these centers around the Y locus in the X-chromo- 
some which may change to y giving a yellow-bodied fly in place of the 
normal gray body or may change to y' when a spot-bodied fly is produced. 
Another system of triple allelomorphs for eye color is located in the 
third chromosome; it consists of the factors for pink and peach eye 
color, and the normal allelomorph of these which is concerned in the 
production of red eyes. A fourth such series of allelomorphs is that 
of the factors for ebony and sooty body color and their normal allelo- 
morph concerned in the production of gray body color. This series is 
also located in the third chromosome. 

Assuming that more than two factors may occupy identical loci in 
homologous chromosomes there are several simple relations which 
must be fulfilled in order to establish the case experimentally. The 

1 The Fi 9s actually have an intermediate eye-color, "white-eosin compound". 



ALLELOMORPHIC RELATIONSHIPS IN MENDELISM 157 

linkage values of such a series of allelomorphs when tested with other 
members of the group to which they belong should be identical. The 
factor for yellow body color is located at the locus 0.0 in the X-chromo- 




FiG. 73. — Forms and hybrids of Paratettix. AA, texanus; BB, leucorotus; CC, leuco- 
thorax; II, nigra notalus. (After Nabours.) 

somes, and displays definite linkage values when tested with any other 
factor belonging in this chromosome. The factor for spot gives exactly 
the same values with all factors with which it has been tested. The 
factors for eosin and white eye color both give one unit of crossing over 



158 



GENETICS IN RELATION TO AGRICULTURE 



with the factor for yellow body color and they give identical linkage 
values with the other factors in this group. Since the factors occupy 
identical loci in the homologus chromosomes not more than two can 
occur in the same individual at the same time. This fact was demon- 
strated in the breeding tests applied above. 

Other cases of multiple allelomorphism are known to occur in a large 
variety of species. In the silkworm there is apparently a series of 





Expectation 
Actual Numbers 

8 

« 


3 
5 

t 

B 

1 


6 
4 

• 
BC 


3 
3 


C 


21 


C 


Expectation 
Actual Numbers 


28.25 
29 




56.5 
53 


28.25 
31 




11 






B 
3 21 


B 

1 


BC 


? 


c 


Expectation 
Actual Numbers 


7;5 
10 


1 

15 
17 


7.5 
3 


9 







A 

1 


1 






• 
AB 




B 

1 


Expectation 
Actual Numbers 


107 




977 






AB 




B 

1 


Expectation 
Actual Numbers 


20 
21 


A 


60 
59 

1 







AB 









1 







B 
I 

231.25 

251 


B 




BC 



462.5 

452 

I 


BC 



231.35 
222 



C 

I 



65 
66 



130 
136 




BC 



65 



1 

5 



AB 




AC 




AC 



111.75 
117 




AC 



6 

BC 



6 


AC 



3 


AD 



37.25 
32 



F2 



Fi 



A— texanus 
B = leuconotus 




AB 

1 c5 from 
the field 





AC 

2 o 8 from 

the field not 

virgin 



Parents 



C = leucothorax 
D= punciofemorata 



Fig. 74. 



-Chart showing results of a continued series of pedigree experiments with 
Paratettix involving types A, B, and C. (After Nahours.) 



multiple allelomorphs concerned in the production of larval patterns. 
As Tanaka has shown there are four larval patterns, moricaud, striped, 
normal, and plain. Each of these colors is allelomorphic to the other 
three and moreover they all apparently display the same linkage values 
with the pair of factors for yellow and white cocoon color. Like the 
multiple systems in Drosophila they give no new types by recombination 
when crossed. 

Multiple Allelomorphs in the Grouse Locust. — A very striking series of 



ALLELOMORPHIC RELATIONSHIPS IN MEN DELI SM 159 

multiple allelomorphs is that concerned with color pattern in Parattetix. 
Nabours has investigated the inheritance of pattern in fourteen races of 
this insect, the grouse locust. Some of these are shown in Fig. 73 and 
also the hybrids between them. It was pointed out in a previous sec- 
tion in this chapter that these races when hybridized give intermediate 
forms in Fi, intermediate in the sense that they display the type 
patterns of both hybrids superimposed one upon the other. In F^ 
they segregate into three types, the two parent types, and the hybrid 
form in the, ratio 1:2:1. 

Nabours has prepared a chart from the data of an extensive breeding 
experiment with some of these forms. It illustrates so admirably the 
type of behavior displayed by multiple allelomorphs that it is given in 
full in Fig. 74. In these experiments separation of B from AB and C 
from AC has not been attempted because the type A exerts very little 
influence on the color pattern of the hybrid. In this chart expected 
results are indicated wherever the ratio of types actually observed is of 
significance. The observed results show excellent agreement with 
expectations. 

The multiple allelomorphs in Parattetix appear to affect the entire 
color pattern of the body and to cause different colors to develop in 
different parts of the body. This, however, is merely another instance 
of the manifold effects of single factors, and furnishes no sound argu- 
ment against the conception of multiple allelomorphs. Furthermore, 
Nabours has discovered at least one modifying factor which can exist 
only with, and in addition to, any of the fourteen multiple allelomorphs 
or their hybrids. 

Multiple Allelomorphs in Maize. — In maize there is apparently a 
remarkable series of multiple allelomorphs concerned in the development 
of red color in the husks, silk, pericarp, and cob. Practically all com- 
binations of these are known in various different varieties of maize, so 
that it is possible to have varieties with red grain, silk, cob, and husk; 
red grain, white silk, white cob, and white husk; or any other com- 
bination whatsoever. When, however, such types are crossed the Fi 
displays a superimposed set of characters, red being dominant; and in 
Fi but three forms appear in the ratio 1:2:1 as with Nabours' locusts, 
namely the two parental types and, if it is different from either of them, 
the hybrid form. This indicates that the Fi hybrids form gametes 
bearing factors determining only the conditions represented in the 
parents. This fact Emerson subjected to direct test by crossing Fi 
hybrids back to varieties lacking the red color in all these parts. In 
one case an Fi plant produced ears which had red cobs and variegated 
red grains. When such a plant was crossed back to a race having white 
cobs and grains, the next generation consisted only of plants which bore 



160 GENETICS IN RELATION TO AGRICULTURE 

ears with white cobs and variegated grains and ears with red cobs and 
white grains. None were produced which bore ears having the F\ 
combination, red cob and variegated grains, and on the other hand 
none were produced showing the reverse recombination, white ears and 
white grains. 

This series of multiple allelomorphs is perhaps the most striking one 
known and displays just as unique relations as does that series in Parat- 
tetix. For considering only red vs. white alone in these characters 
there are sixteen possible combinations which would give pure breeding 
races. Besides this, however, the red, particularly of the pericarp, 
may be modified in many different ways with respect to shade and 
distribution, apparently without altering the relations of the factors 
involved to the allelomorphic system, so that the number of possible 
combinations is considerably greater. Emerson has studied the in- 
heritance of a large number of these types and so far they all may be 
consistently explained on the hypothesis of multiple allelomorphs but 
the data are not as yet extensive enough to establish this interpretation 
beyond any doubt. 

The general nature of multiple allelomorphism is attested to by its 
occurrence in widely separated species of animals and plants. Its 
occurrence in Drosophila, the silkworm, Parattetix, and maize has been 
noted above. Besides these Morgan has pointed out that cases are 
known in rabbits and mice among animals, and in Aquilegia, Lychnis, 
and the bean among plants. In rabbits the factors concerned are three, 
those for self-color, Himalayan pattern, and albinism. In the mouse 
apparently four factors make up a similar system, namely those for 
yellow, black, gray, and gray with white belly. In Aquilegia the system 
has to do with leaf color and three factors are involved, those for green, 
variegated, and yellow leaf color. Shull's case in Lychnis has to do 
with sex-determining factors. In the bean the case is somewhat like 
that in corn but the series is less extensive. The system there as re- 
ported by Emerson is green leaves, green pods; green leaves, yellow 
pods; yellow leaves, yellow pods. 

Morgan has brought together the arguments in favor of multiple 
allelomorphism and the following discussion is based for the most part 
upon his presentation. This discussion will serve in a sense as a summary 
of the material dealing with multiple allelomorphism. 

1. Systems of multiple allelomorphs appear always to affect the same 
character. This fact is readily apparent from a consideration of the cases 
which have been cited above. Beyond this the cases often give a series 
of diminishing intensities with respect to the character affected as for 
example, black, Himalayan, and white in rabbits. On this basis, Pun- 
nett has sought to disprove the validity of the hypothesis of multiple 



ALLELOMORPH I C RELATIONSHIPS IN MEN DELI SM 161 

allelomorphs as applied to the case in rabbits, for although the homo- 
zygous forms give such a series of diminishing intensity of melanic pig- 
ment, nevertheless the heterozygous forms give inconsistences. Black 
by agouti gives agouti-black, but black by yellow gives full black, in 
spite of the fact that yellow is regarded as a lower intensity of pigmenta- 
tion than agouti. 

The argument does not appear to be valid, however, for specific 
relations may still exist among the factors of a system of multiple allelo- 
morphs. Bridges has pointed this out in the case of the eye color series 
red, white, cherry, eosin, tinged, blood and buff eye-color in Drosophila. 
He has discovered a number of factors which modify eosin, one in partic- 
ular called whiting changes eosin to pure white, but does not produce 
any visible effect on the other members of the series. The conception 
of diminishing intensity as applied to multiple allelomorphs is clearly 
not fundamental to the hypothesis. 

2. The behavior in inheritance is different from that which would be 
expected in case different loci in the hereditary system were involved. 
When different loci are involved, each of two different mutant types will 
contain besides its own mutant factor the normal allelomorph of the 
mutant factor of the contrasted type. Consequently on crossing they 
will unite the series of factors present in the original type and give a 
character expression corresponding to that of the original form. Such 
is normally the case in undoubted instances of mutations affecting differ- 
ent loci, but in the case of multiple allelomorphs one or the other of the 
mutant types or an intermediate is produced in F\. When identical 
loci are concerned in two mutations, the hybrid between them will not 
reconstitute the original system, but will contain only the two mutant 
factors at that locus. The character expression of the hybrid therefore 
will depend on the interrelations existing between the mutant factors 
and the rest of the hereditary system rather than on the reuniting of the 
normal allelomorphs of the mutant factors. 

3. There are difficulties in explaining the origin of some of the forms 
on the basis of complete linkage between factors, which disappear on the 
adoption of the hypothesis of multiple allelomorphism. The difficulty 
may be illustrated by a specific case, that of the series red, white, cherry, 
eosin, tinged, blood and buff eye-color in Drosophila. Considering two 
specific instances, cherry and white, both of which arose from red immedi- 
ately, it must follow on the basis of complete linkage that one differs from 
red by one factor and the other by two factors. If red be {CE){CE), then 
cherry, which is recessive to red would be (cE){cE), and white, which is' 
recessive to both red and cherry would be (ce)(ce). This involves the 
assumption that white arose as a result of simultaneous mutations in two 

completely linked factors affecting the same character, a practically 
11 



162 GENETICS IN RELATION TO AGRICULTURE 

inconceivable thing, if viewed from a purely mathematical standpoint, 
unless a special biological mechanism exists which favors such mutations. 
The same difficulties are met with in the case of other systems of multiple 
allelomorphs the origin of which have been observed in pedigree cultures, 
consequently the situation in the above system is not unique. 

4. If a curve of linkage values be plotted in Drosophila for a consider- 
able number of known factors it will be found that the frequencies of 
different values correspond with one another until those displaying 
multiple allelomorphism (or complete linkage) are met with and these 
are far in excess of the number normally to be expected from purely 
mathematical considerations. They are not, therefore, merely the ex- 
tremes of ordinary cases of linkage. 

5. There are no very good reasons why only one sort of change should 
be possible in a given locus in the hereditary material. It is true the 
presence and absence hypothesis does hold that the only difference with 
respect to a given factor is its presence in the hereditary material or 
its absence from it, but there are many reasons why this view at present 
appears untenable. A factor in the hereditary material may well be 
regarded as a complex chemical substance of some kind which maintains 
essential relations with the other factors in the system such that to lose 
it entirely might well disorganize the entire system. But such a com- 
plex chemical substance might well change in many relatively slight ways 
which would modify the particular character in which it is concerned 
in various directions depending upon the specific manner in which the 
factor has been altered. 

When these arguments are considered and the type of cases to which 
it is applied are taken into account, it is apparent that the theory of 
multiple allelomorphism is a useful analytic tool in the solution of a 
certain class of pecuUar Mendehan phenomena. Although some of the 
above cases may prove to be instances of extremely close linkage, never- 
theless for most of them the case is firmly established experimentally, 
and deserves careful consideration from that standpoint. 



CHAPTER IX 
TYPES OF FACTOR INTERACTIONS 

In the present chapter it is proposed to ilhistrate some of the various 
types of relations which exist between different pairs of factors so that 
the student may come to appreciate some of the more complex features 
of factor interaction. 

The System of Aleurone Color Factors in Maize. — It has already 
been pointed out that some particular cases of Mendelian phenomena 
depend for their explanation on the presence of different factors which 
g;ive similar results. These are to be considered as cases of Mendelian 
factor interactions as specifically as those more fully discussed in this 
chapter. Such a condition may be illustrated very satisfactorily by 
the four-factor system for aleurone color which is known to exist in maize. 
The factors involved in this system and their actions are as follows: 

C — a factor for chromogen base. C is necessary for the production of 
any aleurone coloration in maize. Its allelomorph c constantly gives 
white grains. 

R — a factor which when present with C gives a red aleurone color. Its 
allelomorph, r, constantly gives white. 

P — a factor which when present with C and R gives purple aleurone 
color. 

W — a dominant factor for white aleurone color. When it is present 
the grains will be white no matter what other factors may be present. 

With this series of factors, the following homozygous races are ob- 
tainable and have the phenotypic expression here indicated: 

1. WWCCRRPP— white. 9. wwCCRRPP— purple. 

2. WWCCRRpi}— white. 10. wwCCRRpp— red. 

3. WWCCrrPP— white. 11. wwCCrrPP— white. 

4. WWCCrrpp— white. 12. WWCCrrpp— white. 

5. WWccRRPP—white. 13. wwccRRPP— white. 

6. WWccRRpp — white. 14. wwccRRpp — white. 

7. WWccrrPP — white. 15. wwccrrPP — white. 

8. WWccrrpp — white. 16. wwccrrpp — white. 

Of the sixteen pure breeding forms, fourteen are whites, and although 
indistinguishable phenotypically these whites may be separated by 
proper breeding tests. Some of the relations existing between the 
different genotypes have already been dealt with, but the student may 

163 



164 



GENETICS IN RELATION TO AGRICULTURE 



be interested in tracing out others. Thus the presence of a factor for 
dominant white may be demonstrated by crossing with a purple race, 
in which case the grains will be white, if such a factor be present. More- 
over, several of the whites when crossed give colored forms in Fi, thus 
11 X 13, 11 X 14, and 12 X 13 give purple, and 12 X 14 gives red. 
It has already been shown how in case of the presence of the factor for 
dominant white, whites when crossed may give a white Fi and white, 
purple, and red in various proportions in F2. Such is the case for ex- 




FiG. 75. — Comb types in poultry. Single, a; pea, b; rose, c; walnut, d; and breda, e. 

{After Morgan.) 

ample in the cross 1X16 which will give Fi white and F2 in the ratio 220 
white: 27 purple: 9 red. The complex relations here existing between 
only three phenotypes is a very good example of the sort of problems 
which must be solved by experimental genetics. 

Comb -characters in Fowls. — A variety of comb-characters are found 
in the domestic breeds of poultry and Bateson has made these the sub- 
ject of an extensive Mendelian investigation involving the rearing of 
over 12,000 individuals. The comb types involved are shown in Fig. 
75. In this series of characters both rose and pea comb were found 
to be dominant to single comb, and give in F2 simple 3 : 1 ratios. These 
relations obviously indicate that there is a single factor difference be- 



TYPES OF FACTOR INTERACTIONS 



165 



tween rose and single and pea and single, but that the pair of factors 
involved must be different in each case. This is again shown in crosses 
between walnut and rose or pea-comb fowls for such crosses give walnut 
in Fi and 3 : 1 segregation in F2. Accordingly taking walnut as a domi- 
nant type, rose comb may be conceived of as differing from it by the 
factor r, and pea comb by the factor p. Walnut comb would then 
contain the dominant allelomorphs RRPF, and rose comb would be 
of the genetic constitutioi;^ rrPP, and pea comb, RRpp. Since single 
comb again differs by one factor from both rose and pea comb it must 
be of the genetic constitution rrpp. This analysis explains the experi- 
mental results which we have thus far outlined, but the critical test of 





RP 


Rp 


rP 


rp 


RP 


RRPP 
Walnut 


RRPp 
Walnut 


RrPP 

Walnut 


RrPp 

Walnut 


Rp 


RRPp 

Walnut 


RRpp 
Pea 


RrPp 
Walnut 


Rrpp 
Pea 


rP 


RrPP 

Walnut 


RrPp 
Walnut 


rrPP 
Rose 


rrPp 
Rose 


rp 


RrPp 

Walnut 


Rrpp 
Pea 


rrPp 
Rose 


rrpp 
Single 



Fig. 76. — Checkerboard analysis of theoretical expectations in F2 from a cross between 
rose-comb fowl (rrPP) and pea-comb fowl (RRpp). 

the hypothesis lies in the cross rose X pea. This should give walnut, 
RrPp, in Fi and in F2 all four types in the proportions 9 walnut: 3 
rose: 3 pea:l single as shown in the checkerboard in Fig. 76. In 
one series of such experiments Bateson obtained the results shown in 
Table XXX. 



Table XXX. — Inheritance of Comb Type in Fowls. 



Cross 


Walnut Pea | Rose | Single 


RrPp X RrPp 


Observed. . . 
Expected. . . 
Ratio 


279 

312 

9 


132 

104 

3 


99 

104 

3 


45 

35 

1 


RrPp X rrpp j 


Observed. . . 
Expected. . . 
Ratio 


664 
687 

1 


705 

687 

1 


664 

687 

1 


716 

687 

1 



In this table the results of the back cross of Fi walnut to single are 
also given. A comparison of the values given with those expected on 



166 



GENETICS IN RELATION TO AGRICULTURE 



the basis of independent segregation of the factors indicates a fairly 
close correspondence between the two. It may be of some significance, 
however, that walnut and rose are the deficient classes in both cases. 

From the standpoint of factor interaction this case is of interest 
because it shows clearly that the character expression of a given set of 
factors cannot be predicted with certainty from the known character 
expression of some of these factors. It would have been impossible 
to predict from the character expressions involved that rose X pea 
would give walnut-comb fowls or that by recombination of the two 
recessive factors involved a single-comb fowl would result, for these 
two new comb types are totally different from the rose and pea types 
from which they can be derived. The obtaining of new characters of 
this kind by factor recombination is by no means an unusual thing in 
genetic experiments, and is sufficient justification in breeding work for 
testing factor combinations to determine what sort of character ex- 
pression may result from them. 





CRVH 


CrVH 


cRVH 


crVH 


CRVH 


CCRRVVHH 

Violet hairy 


CCRrVVHU 

Violet hairy 


CcRRVVHH 

Violet hairy 


CcRrVVHH 

Violet hairy 


CrVH 


CCRrVVHH 
Violet hairy 


CCrrVVHH 
Cream glabrous 


CcRrVVHH 

Violet hairy 


CcrrVVHH 

Cream glabrous 


cRVH 


CcRRVVHH 
Violet hairy 


CcRrVVHH 
Violet hairy 


ccRRVVHH 
White glabrous 


ccRrVVHH 

White glabrous 


crVH 


CcRrVVHH 
Violet hairy 


CcrrVVHH 
Cream glabrous 


ccRrVVHH 

White glabrous 


ccrrVVHH 
White glabrous 



Fig. 77. 



checkerboard analysis of a cross between two varieties of stocks, white 
glabrous (ccRRVVHH) and cream glabrous (CCrrVVHH). 



Miss Saunders' Factor System in Stocks. — A more complicated case 
of factor interaction as related to character expression has been in- 
vestigated by Miss Saunders in stocks (Matthiola) and has been inter- 
preted in somewhat the following fashion with respect to the factors 
and factor relations therein concerned. 

C — a factor for chromogen base which by itself gives a cream-colored 
flower. Its allelomorph, c, gives white flowers. 

R — a, factor for red coloration, epistatic to C. 

V — a factor for violet coloration epistatic to R. 

H — a factor for the production of hairs on the leaves, active only in the 
presence of C and R. 

The complicated relations existing between these factors are well 



TYPES OF FACTOR INTERACTIONS 



167 



illustrated by the cross white glabrous {ccRRVVHH) X cream glabrous 
(CCrrVVHH). This gives in T^i violet hairy plants (CcRrVVHH) which 
segregate in F2 according to the analysis given in the accompanying 
checkerboard (Fig. 77). The proportions are 9 violet hairy: 3 cream 
glabrous: 4 white glabrous. 

The peculiar feature of these relations is the fact that the factor H 
for hairiness can only act in the presence of C and R. In fact as far as 
the above experiment goes, the hairy condition might be considered as 
merely an extra effect of the interaction of C and R. However, glabrous 
violet plants are known and in these the factor h for the glabrous con- 
dition must be present. When a violet gla])rous {CCRRVVhh) plant 
is crossed with white glabrous {ccRRVVHH) the Fi again is violet hairy 
{CcRRVVHh), this time because the factor for hairiness is brought 
in by the white plant, and in F2 the segregation is as indicated in the 





CRVH 


CRVh 


cRVH 


cRVh 


CRVH 


CCRRVVHH 

Violet hairy 


CCRRVVHh 

Violet hairy 


CcRRVVHH 

Violet hairy 


CcRRVVHh 

Violet hairy 


CRVh 


CCRRVVHh 

Violet hairy 


CCRRVVhh 
Violet glabrous 


CcRRVVHh 

Violet hairy 


CcRRVVhh 
Violet glabrous 


cRVH 


CcRRVVHH 
Violet hairy 


CcRRVVHh 
Violet hairy 


ccRRVVHH 
White glabrous 


ccRRVVHh 
White glabrous 


cRVh 


CcRRVVHh 
Violet hairy 


CcRRVVhh 
Violet glabrous 


ccRRVVHh 
White glabrous 


ccRRVVhh 
White glabrous 



Fig. 78.- 



-Fi checkerboard analysis of a cross between violet glabrous {CCRRVVhh) and 
white glabrous {ccRRVVHH) stocks. 



checkerboard in Fig. 78. The phenotypic ratio obtained this time is 
9 violet hairy: 3 violet glabrous: 4 white glabrous. 

This analysis not only adequately accounts for the phenomena as 
given above, but it also accounts for the F3 results and the various types 
of results that are obtained by mating other genotypes. In addition 
Miss Saunders found that when purple or white incana were mated to 
cream of the type above, the entire series of forms recorded for the 
previous white X cream mating were obtained and in addition cream 
hairy and cream glabrous. This at first sight appears to contradict 
the hypothesis that no cream or white hairy forms are possible. But 
closer examination has revealed the fact that white incana, which is 
itself hairy is in reality a colored form, i.e., possesses the factors C and 
R. This is shown by the fact that a slight tinge develops in flowers of 
this variety on fading, and in the F2 from a cross of this form with cream 




168 GENETICS IN RELATION TO AGRICULTURE 

glabrous, those whites which tinge on fading are hairy and those which 
show no sign of coloration on fading are glabrous. The apparent diffi- 
culty is therefore merely due to the fact that some plants which possess 
C and R are still white on account of the action of other factors. 

Altenburg and MuUer's Truncate -winged Drosophila. — An even 
more complicated case of factor interaction is that concerned in the 
production of truncate wings in Drosophila (Fig. 79). The factors 
here involved appear to be the following: 

t — a factor for truncate wings. It is a recessive factor located in the 
second chromosome, and without this factor the truncate 
wing character cannot appear. 

^1 — a factor which intensifies the expression of the 
truncate wing character, but which is not absolutely 
essential. This factor is located in the first chromosome. 
tz — another factor which intensifies the expression 
of the truncate wing, but is not absolutely essential to it. 
B' — -the dominant factor for bar eyes which in ad- 
dition acts as an intensifier of truncate. This is a first 

chromosome factor. 

line drawing of a h — a factor for black body color located in the second 

truncate- winged chromosome. This factor has such an influence that 
ter Morgan.) flies of the constitution {bT){bt) or even {Bt){hT) may 

display the truncate wing character. 
The truncate wing character was particularly baffling on account of 
the extraordinary relations which it displayed both in hybridization and 
in selected strains. In hybridization instead of a 3:1 ratio of long to 
truncate wing the ratio was about 7 : 1 and in selected strains even after 
100 generations of selection there were still about 5 per cent, of long winged 
flies. That these long winged flies were different genetically from 
the truncate winged flies was shown by breeding tests for in such tests 
they did not produce as high a percentage of truncate winged flies as 
did those which had truncate wings. By means of linkage relations, 
however, it was possible to determine the factors concerned, and their 
specific effects. Particularly noteworthy is the fact that the factor B' 
for bar eyes acts as an intensifier for truncate, thus providing an analo- 
gous case to that in stocks where the color factors are necessary for the 
action of the factor for hairiness. No less interesting is the affect of h, 
for it was found that this factor, whether homozygous or heterozygous, 
changed the dominance relations in the allelomorphic pair Tt, so that 
the truncate wing character is expressed in such individuals when hetero- 
zygous for t. Furthermore since truncate appears more readily in the 
female than in the male it would appear that the sex factors also act as 
intensifiers. 



TYPES OF FACTOR INTERACTIONS 



169 



The important point involved in this case, however, is the ingenious 
way in which the investigators made use of the hnkage relations and the 
known fact that crossing-over does not occur in the male in order to study 
these factors, particularly with reference to their constancy, since they 
are variable in phenotypic expression. They took a truncate male which 





9{bT){pT,)X 


(bT)ipT,)X 


{bT)ibT){pT,)XX 
black pink 9 
long 


{bT)iPt,)X 


(bT){bT){Ph){pT,)XX 
black red 9 
long 


{Bl){pT,)X 


{Bt){bT)iPt,)ipT,)XX 
Gray pink 9 
long or truncate 


{Bt)iPl^)X 


{Bl){bT)iPk)ipT,)XX 
Red gray 9 
long or truncate 


ibT)(pT,)X 


{bT){bT){pT,){pT,)XX 

Black pink cf 

long 


{bT){Pt,)Y 


{bT){bT){Ph){pT,)XY 
Black red cT 
long 


{Bt)ipT,)Y 


{BT)(JbT){pT,){pT,)XY 

Gray pink cT 

long or truncate 


(Bt){Ph)Y 


{Bt)ibT){PT,)ipT,)XY 

Gray red cf 

long or truncate 



Fig. 80. — Checkerboard analy.sis of Fi generation obtained by mating an Fi male Droso- 
phila of the constitution {hT){Bt)(pTi){Pt:i)XY with a pink l)laok long female. 



contained the truncate factor and also the truncate intensifier of the 
third chromosome and mated it to a long-winged black-bodied female with 
pink eyes. The genetic constitution of the truncate male with respect 
to the factors involved was {Bt) (Bt) (Ph) {Ph)X Y, and the contrasted black 
female was {bT){bT){pT3)ij)T3)XX. A male from such a cross is of the 



170 GENETICS IN RELATION TO AGRICULTURE 

genetic constitution {hT){Bt){pT3)(Pt3)XY, and since no crossing-over 
occurs in the male it produces the following series of gametes: 

{bT){pT,)X {bT){pT,)Y 

{bT){Pt,)X {bT){Ph)Y 

{Bh){pT,)X {Bh){pT^)Y 

{Bh){Ph)X {Bh){pT,)Y 

When, therefore, such an Fi male is mated back to a black long pink 
female the results are as recorded in the checkerboard in Fig. 80. Of 
the male flies only the gray reds bear both the factors t and ^3. Such 
flies are long or truncate winged, but they should behave in the same fash- 
ion in further breeding tests unless the factors themselves are variable. 
Actually it was found that continued breeding back of these gray red 
males to black pink females gives approximately the same proportions 
of truncate to long in every generation. This method of taking advan- 
tage of the linkage relations and using the pink factor so that a given 
genotype could be determined without fail has in this series of experiments 
been the means of analyzing a case which otherwise would have baffled 
investigation, for the results clearly point to the fact that the genotypic 
differences which exist between the long and truncate flies of a selected 
culture are due to the fact that the lower vitality of truncated flies homo- 
zygous for the three factors directly concerned in the expression of this 
character favors the survival of heterozygous individuals, and it is, there- 
fore, practically impossible to secure a strain of truncate winged flies 
which will breed true. 

The Factor Explanation of Reversion. — Many phenomena included 
under the term reversion can be explained satisfactorily as instances of 
complex factor interaction. Reversion in general is a term applied to 
sudden return to an ancient, generally wild form, whether by hybridiza- 
tion or from other causes. 

The Mendelian explanation of reversion is most simply illustrated in 
Drosophila, for in Drosophila the relation of any particular form to the 
wild type is known accurately. Thus for example a form of Drosophila 
with miniature wings arose as a mutation directly from the long-wing 
type. Likewise several other wing characters have arisen from the long- 
wing type by a single mutation, among them vestigial wings. When now 
a vestigial-winged female is mated to a miniature male, the progeny all 
have long wings. This phenomenon may be explained by the fact that 
in a vestigial fly, a mutation has occurred in the locus V, which changed 
it to V without affecting the normal allelomorph of the miniature factor. 
Similarly the miniature fly bears the normal allelomorph of the vestigial 
factor, so that when the two are mated the original series of factors of the 
long-winged type is reunited and consequently the characters of the 
original wild form are reproduced. This is the principle on which rever- 



TYPES OF FACTOR INTERACTIONS 



171 



sion in hybridization depends, and other cases differ from this one only 
in the number of factor differences involved. 

Among the most notable cases of reversion arc those which Darwin 
describes in pigeons and fowls. Darwin regarded these throw-backs to 
wild types which he obtained by crossing various breeds of pigeons as 
important evidence of phyletic origin, and largely on the basis of this 
evidence concluded that the many varied modern breeds of pigeons are 
monophyletic in origin, that they are all derived from a single wild species. 
This species is the Wild Rock Pigeon, Columha livia, and in the wild it 
has an extended range over Europe, Abyssinia, India, and Japan. Even 
in the wild state it is variable, but under domestication breeds have 
been developed which show truly remarkable differences, and Darwin 
has described and illustrated these with great care. 

The hybridization experiments which Darwin conducted with domes- 
ticated breeds of pigeons were undertaken for the purpose of establishing 
relationship to the Wild Rock Pigeon. The phenomenon of throwing 
blue in pigeons is an exceedingly common one, but Darwin conducted 
experiments with breeds which had been bred for many generations and 
rarely, if ever, gave blue birds. Cole has summarized the results of one 
of his experiments about as in Fig. 81 : 



Barb cf X Spot 9 



(Self black) 



(White with red 
spot on forehead 
and red tail). 



Fi Barb-spot 
(Black or brown with some 
white splashes). 



Barb <f X Fantail 9 



(Self black) 



(Self white) 



i 
Fi Barb-fantail 
(Black with some white 
flights and tail feathers). 



i 
Mongrel Barb-spot cf 
(Bird without a trace of 
blue, generation not stated). 



X 



i 
Mongrel Barb-fantail 9 
(Bird without a trace of 
blue, generation not stated). 



Fig. 81.- 



Reversionary Blue 
(One bird, head tinted with red) 

-Pigeon breeding experiment resulting in reversion. 



(After Cole.) 



Evidently on a factorial basis this case involves a complicated recom- 
bination of factors, and it can only be said that the Barb, Spot, and Fan- 
tail breeds which Darwin used differed from the wild pigeon in different 
factors and that in this experiment the original set of factors which is 
responsible for the blue color of the Wild Rock Pigeon was reconstituted. 
Darwin points out that this bird differed only in a few unimportant de- 



172 



GENETICS IN RELATION TO AGRICULTURE 



tails from the color pattern of the Wild Rock Pigeon from the Shetland 
Islands. 

The data which Darwin presented while giving qualitative evidence 
of recombination of factors do not provide quantitative data on which to 
base a Mendelian analysis. Staples-Browne, however, has presented 
a case which admits of more definite analysis. He crossed a Black- 
barb and a White Fantail and obtained black birds in Fi. In F2 he 
obtained the results which are tabulated in Table XXXI. Cole has pro- 
posed the following factor analysis for the principal colors in pigeons: 

B — a factor epistatic to the series of factors which produce red in 
pigeons, and giving black pigmentation. The allelomorph 6 then results 
in red pigmentation. 

S — a factor for extension of pigment which appears to act on B only. 
BS gives black birds, whereas Bs gives birds in which the black pigment 
is aggregated into clumps in the barbules, the so-called blue pigeons. 

T — a factor which gives black tail feathers when B is present. The 
allelomorph h under similar conditions gives blue tailed birds. 

W — a series of factors for white pigmentation which Cole designates 
Wi, W2, Ws, Wn- By Wn is designated simply an undetermined number 
of these white factors which may affect different portions of the plumage. 
Likewise by Wn is designated the recessive allelomorphs of these white 
factors, birds with such a genetic constitution being self-colored. 



Table XXXI. — F2 Results from Black Barb X White Fantail Pigeons 



Classes 



Black 
B 



Red 
b 



Black 

tail 

T 



Blue 

tail 

t 



Pigmented 
(self) 



Splashed 



Little 
white 



Much 
white 



White 
self 



5 Black (self) 

10 Black (white feathers) 

2 Blue (self) 

2 Blue (white feathers) . . 

5 Red (white feathers). . . 
2 White (black feathers) . 
2 White (red feathers). . . . 

6 White (self) 

Total observed 

Expected 



5 
10 

2 

2 



21 
21 



5 
10 



15 
14 



10 

2 

2 



14 
? 



With these factors the Black Barb would be BBSSTTw^Wn and the 
White Fantail hhSSttWnWn. The Fi of such a cross, BbSSTtWnW^ 
would be black more or less splashed with white as were those obtained 
in the experiment, and in F2, a series of forms would be obtained de- 



TYPES OF FACTOR INTERACTIONS 173 

pending on the dihybrid nature of the BhTt portion of the pedigree and 
the number of W factors present. Disregarding these latter the ratio 
in F2 should be 9 self black :3 black with blue tails :4 red, since the 
action of the factor T has not been determined in red birds. Of birds 
which might be classified in these categories there were actually 15 
self-black: 4 black with blue tail: 5 red, a very close agreement for 
such small numbers. Moreover, collecting those with factors B and h 
which should give a 3 :1 ratio, the numbers are 21 :7 and similarly the 
numbers for black tail and blue tail are 15 :4. The ratios, therefore, 
are in close agreement. 

It should be noted that the above "blues" are not really the blue of 
the Wild Rock Pigeon but are blacks with blue tails. We can, however, 
understand how a blue pigeon might arise from mating black to white 
for it would only be necessary to employ a white of the genetic con- 
stitution hhssttWnWn to achieve this result. It is thus that complex 
cases of reversion may be explained by reconstitution of the combination 
of factors present in the original wild form. 



CHAPTER X 
FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 

By quantitative inheritance is meant inheritance which has to do 
with the size of organisms or parts of organisms, with the number of 
certain parts, or at times with shapes and forms as related to size. The 
category of quantitative inheritance is one of convenience only and is not 
separated sharply from other forms of inheritance. It is, however, in 
general notable for its complexity and difficulty of precise analysis, and 
these features of it have resulted in diverse interpretations, some in 
harmony with the Mendelian principles which have been discussed 
up to this time, and others which call upon auxiliary hypotheses for 
aid. In this discussion, we shall begin with simple cases of size in- 
heritance and proceed from them to others more complex. Throughout 
the attempt will be made to develop a consistent explanation for the 
phenomena, and one which is in harmony with the general explanation 
advanced in Mendelian heredity. 

Mendel's original experiments dealt with one case of size inheritance. 
When a tall pea is crossed with a dwarf pea, the Fi generation consists 
entirely of tall peas, and in the F2 progeny there are approximately 3 
tall peas : 1 dwarf. Further' tests in the F3 generation showed that the 
dwarfs breed true to that character, they give only dwarf progeny. 
One-third of the tails, also, breed true to the tall character, but the 
other two-thirds give progenies which display segregation into tall 
and dwarf in the ratio originally obtained in the F2 generation, namely 
3 tall : 1 dwarf. The results throughout are, therefore, in harmony 
with the assumption that there is a single factor difference between tall 
and dwarf peas and that the factor for tall is completely dominant over 
the dwarf factor. This case we have treated in detail in Chapter V. 

Results similar to this are not uncommon in studies in size inheritance 
involving differences between tall and dwarf races. They have been 
reported for tall and dwarf tomatoes, sweet peas, maize, beans, snap- 
dragons, etc. In the case of beans, Emerson has pointed out that the 
differentiating factors are factors for indeterminate as opposed to a deter- 
minate habit of growth. In the dwarf or bush type of bean, illustrated 
by Fig. 82 (right), the axis of the plant is terminated after the formation 
of from four to eight nodes by an inflorescence. Pole or tall beans, as is 
shown on the left in Fig. 82, do not have such a terminal inflorescence 

174 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 175 



and consequently continue growth until checked by unfavorable external 
conditions or by the drain of seed production. It is, therefore, possible 
in this case to state definitely upon what a size difference depends. 

In sweet peas there are two distinct dwarf forms which display a 
simple type of inheritance when crossed with tall forms. One of these 
types is the Cupid sweet pea which originally arose as a mutation from 
Emily Henderson, a tall white variety. The Cupid sweet pea is a very 
dwarf procumbent type which produces no erect stems. When crossed 
with tall varieties it gives tall plants in F] and in F2 segregation into 




Fig. 82. — (right) Young bush bean plant showing determinate habit of growth. The 
axis is terminated by a flower cluster; (left) young pole bean plant showing indeterminate 
habit of growth. The flowers are all in the axils of the leaves. (After Emerson.) 

tall and dwarf in the ratio 3:1. The other restricted form is the bush 
sweet pea. It is characterized by a profuse production of thin, wiry 
branches which intertwine and form a bush, sometimes three and a 
half feet high. Bush crossed with tall gives in Fy tall plants and in F2, 3 
tall : 1 bush. The bush sweet pea, therefore, like the Cupid differs from 
the tall sweet pea in a single genetic factor, and in both cases the tall 
form is completely dominant. 

An interesting situation arises when bush sweet peas are crossed with 
Cupid sweet peas. From our knowledge of the inheritance exhibited 
by these two forms when crossed with tall, we may assume as in previous 
cases that in the appearance of the Cupid mutation there was a change 



176 



GENETICS IN RELATION TO AGRICULTURE 



in some locus concerned in the development of height, such that a low 
prostrate form was produced rather than the normal tall form. We may 
call this locus D and its mutated condition in Cupid sweet peas d (dwarf). 
In the bush sweet pea a change must have occurred in some other locus 
concerned in the development of height such that the bush type was 
produced rather than the normal tall sparsely branched type. This 
locus in the germinal material may be called B, and its mutated con- 
dition responsible for the bush type of growth, h (bush). Considering 



DDBB 


DDBb 


DdBB 


DdBb 


tall 


tall 


tall 


tall 


all tall 


3 tall : 1 bush 


3 tall : 1 Cupid 


9 tall : 3 bush : 

3 Cupid : 
1 bush-Cupid 


DDBb 


DDbb 


DdBb 


Ddbb 


3 tall : 1 bush 


all bush 


9 tall : 3 bush: 


3 bush: 






3 Cupid : 


1 bush-Cupid 






1 bush-Cupid 




DdBB 


DdBb 


ddBB 


ddBb 


tall 


tall 


Cupid 


Cupid 


3 tall : 1 Cupid 


9 tall: 3 bush: 


all Cupid 


3 Cupid : 




3 Cupid: 




1 bush-Cupid 




1 bush-Cupid 






DdBb 


Ddbb 


ddBb 


ddbb 


tall 


bush 


Cupid 


bush-Cupid 


3 Cupid : 


3 bush: 


3 Cupid : 


all bush-Cupid 


1 bush-Cupid 


1 bush-Cupid 


1 bush-Cupid 





Fig. 83. — Checkerboard analysis of the Fi population resulting from a cross between bush 

and Cupid sweet peas. 

both loci, the tall form must have the genetic constitution DDBB, since 
a change in either one of these factors results in some other type than 
the normal tall. The bush will then obviously be DDbb, since it shows 
only a single factor difference from tall, and likewise the Cupid sweet 
pea will be ddBB. When, therefore, bush is crossed with Cupid, the 
genetic formula for the Fi is DdBb. Since the factor combination nec- 
essary for the production of tall plants is reconstituted in this cross, the 
Fi plants are all tall. In F2 segregation takes place in accordance with 
expectations based on independent distribution of the two pairs of 
allelomorphs as shown in the accompanying checkerboard in Fig. 83. 

The F2 segregation here displayed is in agreement with the usual 
dihybrid ratio when dominance is complete. There are 9 tall : 3 bush : 3 
Cupid :1 bush-Cupid. In the bush-Cupid a new combination of factors 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 177 

is involved, and the bush-Cupid correspondingly combines the charac- 
teristics of the bush and Cupid sweet peas. Like the Cupid sweet pea 
it is a very dwarf form, but it is erect and much branched like the bush 
sweet pea. In this particular instance, therefore, the combination of 
recessive factors results in a combination of the characters which these 
factors ordinarily determine. The segregation gives four distinct types 
in F2 which are easily distinguishable one from another at maturity, and 
which behave in characteristic manners in the following Fs generation 
as is indicated by the accompanying checkerboard. The case, while 
more complex than those in which only one factor difference is responsible 
for the character differences, is still simple and readily analyzed by pedi- 
gree culture methods. Figs. 123 and 124 illustrate these forms of sweet 
peas. 

In Drosophila there is at least one size difference dependent upon a 
simple factor difference. This factor belongs in the third group and 
bears the same relations to the other factors of the genetic system as are 
displayed by any other locus. The factor dwarf and the corresponding 
character are in every respect strictly analogous to true qualitative factors. 
In general the facts of heredity with regard to the simple size differences 
which we have noted are in agreement with the interpretation that they 
have arisen by some mutation in a single locus in the normal form and 
therefore naturally enough display simple monohybrid inheritance. 
In the case of Lima beans the origin of the bush type by mutation was 
definitely observed and similarly the Cupid sweet pea was discovered 
among a population of tall forms. There is no reason, therefore, for 
advancing any different explanations than those here given for size differ- 
ences of this type. 

There are other size differences which are not particularly different 
from those above noted except that they are in the opposite direction. 
A case in point is the mutation in tobacco discovered by East and Hayes. 
This mutation is one in which the habit of growth has been changed from 
the ordinary determinate type in which an inflorescence terminates the 
axis at a relatively invariable stage of elongation to one in which the axis 
continues growing and producing leaves for a considerably longer period 
of time. This giant type of tobacco is a recessive form, and like the size 
differences above noted represents a single factor difference from the 
normal type (see Fig. 150). In Drosophila there is a factor for giant size 
in the third group which gives rise to a form several times larger than the 
normal type, and like the factor for dwarf, which is in the same chromo- 
some, there is no difference between it and its relation to other factors 
and the relation of these factors to each other. These facts like those 
which have been discussed above merely serve to emphasize the fact that 
size differences, some of them extremely wide, may depend upon simple 



178 



GENETICS IN RELATION TO AGRICULTURE 




Fig. 84. — Outline diagram of a cotton leaf. The cotton-leaf factor 

{After Leake.) 



a — b 




Fig. 85. — Typical cotton leaves; upper left, with leaf factor less than 2; below, with leaf 
factor greater than 3; upper right, with intermediate leaf factor, approximately 2.5. (After 
Leake.) 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 179 



factor differences, differing, therefore, in no respect from those other 
MendeUan differences concerning which no question is raised. 

The Cotton Leaf Factor. — Leake has investigated the inheritance of 
the so-called cotton leaf factor. The results of his investigations are 
given here in some detail because they illustrate very well the simplest 
expression of the most common type of quantitative inheritance. The 
so-called cotton leaf factor is essentially a length breadth index of the 
terminal lobe of the leaf. It is obtained by dividing the difference 



; -L 1- H- - 




1 


"1 


1 


1- - . ' 1 .- 




' ' 


' 1 I 


r 




"I 




^2 T 1 4 IL ^ 4 ' 


II II ^i . r.l 




























^1 I \ : W-Jz 


I ^ 1 1 r 














s 




c 


2 


fe 












'1 




:i t^^^ W 1 41 


n: I iiZL II.. I. ..-II 



Leaf- Factor 1.1 
Values 



1.6 



3.6 



4,1 



2.1 ~ 2.6 3.1 

Type 4 x Type 8 

Fig. 86. — Distribution of parental, Fi, and Fo plants with respect to leaf-factor values. 

(After Leake.) 

between the two measurements a and h in the accompanying diagram 
(Fig. 84) by the width c of the terminal lobe, or expressed algebraically 

it is the value of the expression Although there are variations 

in this value for different leaves on a single plant, Leake found that races 
might be obtained which were characterized by relatively constant leaf 
factors. Leake crossed one of these races with a mean leaf factor of 
L52 with a race the mean leaf factor of which was 3.47. The mean leaf 



180 



GENETICS IN RELATION TO AGRICULTURE 



factor of 28 Fi plants obtained from this cross was 2.39, a value 
which does not differ significantly from the average leaf factor of the 
two parents, 2.49. Fig. 85 illustrates three leaves representing the three 
types concerned in this cross. From the Fi plants of this cross 195 F2 
plants were grown, and their leaf factors were found to be distributed as 
shown in Fig. 86. The striking feature of this F2 distribution is that, 
although practically all values for the leaf factor between the two parental 
extremes are represented, these frequencies of values give a trimodal distri- 
bution with modes which correspond approximately to those of the parents 
and the Fi. If these F2 individuals be divided into three portions, as 
shown in Table XXXII corresponding to the two parents and the Fi, the 
mean value for the upper group is 3.42 as compared with the mean value 
of 3.47 for the corresponding parental type, the mean value of the mid 
group is 2.59 corresponding to 2.39 for the Fi, and the lower group has a 
mean value of 1.66 as compared with 1.52 for its corresponding parental 
group. In the upper group there were 46 plants, in the mid group 102, 
and in the lower group 47, a satisfactory agreement with a 1:2:1 ratio. 



Table XXXIl. 



-Re-appearance of Parental Values in the F2 Offspring 

{After Leake). 



Pi 




aa 
Parent type No. 4 




AA 
Parent type No. 8 




Leaf factor 


1.52 


Mean 2.49 


3.47 


Fi 


28 plants 




Aa 

Mean leaf factor 

2.39 






Classes 


Lower 


Middle 


Upper 


F2 


Leaf factor 


Less than 2 


Greater than 2 
Less than 3 


Greater than 3 




No. of individuals 

Ratio 

Mean leaf factor 


47 

laa 

1.66 


102 

2.2 aA 
2.59 


46 

1 AA 
3.42 



This case then gives a simple expression of the general phenomena 
assumed to be operative in the multiple factor theory of size inheritance. 
If it be assumed that a single factor difference be here operative, then the 
small parent might be represented as aa and the large parent as AA. 
Plants of the genetic constitution aa then fluctuate for their leaf factor 
around 1.52 as a mean, whereas plants of the genetic constitution A A 
fluctuate around 3.47. When these two races are crossed the Fi is 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 181 

intermediate, for in quantitative inheritance in general the factors ap- 
parently do not display dominance but the phenotypic expression of Aa 
is approximately equal to the average of that of the two parents. If 
this idea be correct the F2 should consist in this case of lAA: 2Aa : laa, 
which has been shown to be approximately true, if the boundaries shown 
in Fig. 86 be accepted. It is to be noted, however, even for as simple a 
case as this that the two parents and the Fi each have their typical ranges 
of fluctuation. Considering the parental and Fi distributions, the upper 
limits of the smaller parent run into the lower portion of the range of the 
Fi, and the lower limits of the larger parent run into the upper limits of 
the Fi. It follows that in F2, even if there should be only a single factor 
difference between the two parents, the F2 classes would overlap. Con- 
sequently some of the plants assumed to belong to the lower class and 
also some of those assumed to belong to the upper class really belong to 
the middle group, and some of those arbitrarily included in the middle 
class belong either to the upper or to the lower class. 

The only accurate method of classifying those individuals lying near 
the boundaries of the classes is to test them by growing their F3 progenies. 
If they belong to the upper or lower classes they should then give uni- 
modal distributions corresponding to the parental distributions, whereas 
if they belong in the middle class, they should give trimodal distributions 
corresponding to the distribution obtained in F2. In another case 
Leake has endeavored to apply this test to an entire F3 distribution, and 
although the evidence indicates some discrepancies probably due to the 
presence of minor factor differences, nevertheless the agreement is such 
as to lend support to the idea of a single main factor difference. 

Most size differences however are not so readily interpreted as this 
one, apparently because a larger number of factors is concerned in them. 
The general statement with regard to such size differences is that two 
contrasted races, each of which displays a certain characteristic amount 
of variability when grown in the pure line, when crossed produce an Fi 
intermediate between the two parents and no more variable than either 
of them. The F2 from such a hybrid when grown in large populations 
displays on an average an intermediate position, but some few indi- 
viduals at either extreme approach the sizes of the parents, and between 
these extremes lies a continuous series of forms, in distribution usually 
approximating a normal curve. The evidence of segregation here is the 
increased variability in the second generation, and subsequent genera- 
tions display a similar conformity to such an interpretation. 

This type of inheritance may best be illustrated by a typical example, 
but one which has, perhaps, been more thoroughly investigated than 
any other, namely the inheritance of length of corolla in tobacco. East 
has investigated the inheritance of length of corolla in crosses between 



182 



GENETICS IN RELATION TO AGRICULTURE 



two varieties of tobacco of the species Nicotiana longiflora. The smaller 
of these two varieties has a tube length of about 40 mm., whereas the 
contrasted variety bears flowers the tube length of which is over twice 
as great, namely about 93 mm. The two varieties had been self-fertil- 
ized for a number of generations preceding hybridization, and since it 
can be demonstrated that continuous self-fertilization tends to reduce a 
variety to a homozygous condition, it is fair to conclude that the parents 
represented varieties homozygous for nearly, if not quite, all their fac- 
tors. We are not surprised, there- 
fore, to find that they display only 
a slight variability in flower size. 
This slight variability is to be con- 
sidered merely an evidence of the 
influence of external conditions and 
of inherent variability in character 
expression and not of internal 
heterozygosity, for there is a limit 
below which it is apparently im- 
possible to force the reduction in 
variability of any given character. 
In this case the accompanying table 
which has been reproduced from 
East in its entirety will serve as the 
material for the following discussion. 
It will be seen in Table XXXIII 
that when the two varieties were 
crossed the Fi distribution occupied 
a position midway between the two 
parents. The number of plants 
grown was somewhat larger than that 
for the parents, consequently the 
range covered by the Fi distribution is slightly greater, but calcula- 
tions of the coefficient of variability show that the variability of 
the Fi is only slightly and not significantly greater than that of 
the smaller flowered parent. When we look at the F^ from such a 
cross, we find that although it, like the Fi, occupies an intermediate 
position, the range has been doubled and this in spite of the fact that the 
population contained only a few more individuals than that of the F}. 
This increased variability is borne out by calculations of the coefficients of 
variability which are over twice as great for Fo as for Fj. That the in- 
creased variability in Fo is the result of genetic segregation of some sort 
is shown by the distributions of F3 families. They are strikingly different 
from each other in their position on the range, and in the variability which 
they display, as is shown clearly in the table. 




Fig. 87. — Average flowers of two 
varieties of Nicotiana lovgiflora with an 
average flower of the Fi from a cross be- 
tween them in the middle. {After East.) 



FACTOR RELATION'S IN QUANTITATIVE INHERITANCE 183 



Table XXXIII. — Frequency Distkiuutions for Corolla Length in a Cross 
BETWEEN Varieties of Nicoliana longiflora Cav. {After East) 



Designation 
No. 



Class centers in millimeters 



34 37 40 43 46 49152 55158 61 64,67i70 73 76 79182,85 88 91 94 971100 



383. 
383. 
383. 
330. 
330. 
330. 



1911 
1912 
1913 
1911 
1912 
1913 

383 X330 1911 

(383X330)1 1912 

(383X330)2 1912 

(383X330)1-1 1913 

(383 X 330)1-2 1913 

(383 X 330)1-3 1913 

(383X330)1-4 1913 

(383 X 330)2-1 1913 

(383 X330)2-3 11913 

(383X330)2-4 |l913 

(383X330)2-5 1913 

(383X330)2-6 1 1913 

(383 X330)l-2-l 'l914 

(383 X 330)l-.3-l 1914 

(383 X 330)2-6-1 1914 

(383X330)2-6-2 '1914 

(383X330)1-3-1-1 jl915 

( 383 X 330)2-6-2-1 1915 



75 40 3 

18 62 37 
24 37 31 38 
4 20 25 59 
7 1 



25 16 
35 27 
41 19 



33 43 
16 20 
2139 



34 20 
32 41 
39 32 



That the results of expenments in size inheritance may be explained 
by a multiple factor hypothesis is apparent from the explanation which 
follows! For the sake of simplicity we will assume that two races A = 
50 and B = 100 differ by five pairs of genetic factors which display an 
equal effect in size production. The genetic formula for Race A may 
be represented by aahhccddee; and the contrasted Race B by AABBCCDD- 
EE. We assume that the factors display no dominance, that their effect 
is equal and cumulative, and that a dominant factor gives a character 
expression greater by 5 than the corresponding recessive factor. By 
crossing two such races an Fi of the genetic constitution AaBbCcDdEe 
is obtained, which on the above assumptions has a size equal to 75. 
Selfing such a hybrid we would secure, in case these factors displayed 
independent segregation, the following series of phenotypes: 

50 55 60 65 70 75 80 85 90 95 100 
1 10 45 120 210 252 210 120 45 10 1 

These values are merely the coefficients obtained by expanding the 
binomial (a + 6).^" If these values be plotted, they give an approxima- 
tion to the usual form of normal variability curve as shown by the 
polygon representing expansion of this binomial in Fig. 15, and this 



184 GENETICS IN RELATION TO AGRICULTURE 

in general is the type of curve obtained in segregation in quantita- 
tive inheritance. The increased variabiUty in F2 is, therefore, con- 
sistently explainable on the basis of segregation of size factors which lack 
dominance and which display cumulative effects. 

However, in the above study of flower size inheritance the parental 
forms were not recovered in F^. Elsewhere we have adopted a chromo- 
some explanation of heredity, consequently we must inquire what 
chromosome conditions appear to exist in tobacco. So far as known 
the number of chromosomes in Nicotiana is forty-eight. With such a 
large number of chromosomes a duplication of the exact chromosome 
content of each grandparent, assuming that no crossing-over occurred, 
would take place only once in about 365 million millions of i^2 individuals. 
Consequently, if a differentiating size factor be assumed to exist in each 
pair of chromosomes, the reappearance of the grandparental forms on 
the assumptions outlined above would be practically inconceivable. 
It is, however, possible from the data at hand to approximate roughly 
the probable ratio of occurrence of the grandparental forms in F2 popula- 
tions. Assuming that the class distribution in Fo is of the type of the 
normal probability curve, then the larger the number of individuals 
grown in F2, the greater will be the class range over which the distribu- 
tion extends. In this particular flower size problem the average mean 
of the smaller flowered parent is 40.54 mm., and of the larger flowered 
parent 93.30 mm. Half the difference between the means of the two 
parents, therefore, amounts to 26.38 mm. Our problem is to determine 
what proportion of the individuals in an F2 population lie beyond the 
limits set by the value Mp^ ± 26.38 mm., where Mp^ is the value of 
the mean for the F2 population. The mean of one F^ population is 
67.51 mm., and its standard deviation, 5.91 mm. Now by mathematical 
methods it is possible when the standard deviation of a normal prob- 
ability curve is known to determine what proportion of the area lying 
under the curve is within or outside of any assigned limits. If we apply 
these methods to the problem here set, we find that the part of the curve 

26 38 
lying outside the boundaries, -^-5^ = ± 4.46o-, is equal to 0.00080 per 

cent, of the total area under the curve. Since a parental value might as 
often fall short of these modal limits as exceed them, we may fix twice 
this value as that marking off the parental portion of the curve. It 
would, therefore, be necessary to grow some 62,500 individuals in order 
to recover the parental forms in such an experiment as this. Consider- 
ing the other F2 population with a standard deviation of 6.79 mm., the 
limits in this case expressed in terms of the standard deviation are 

26 38 

„ ' „ = 3.88(t: therefore 0.010 per cent, of the curve lies outside the 
6.79 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 185 

indicated limits. This would show that it would be necessary to 
grow about 5,000 individuals in order to recover the parental form. 
The values differ strikingly but they give a rough idea of the ratio of 
occurence of parental forms in F2 populations of this kind. 

East has pointed out that there are about eight mathematical re- 
quirements many of them independent which must be fulfilled in order 
to establish the validity of the multiple factor theory of size inheritance. 

1. Crosses between individuals which from long-continued self- 
fertilization or other close inbreeding approach a homozygous condition 
should give Fi populations comparable to the parental races in uniformity. 

Continued self-fertilization tends very quickly to reduce a race to a 
condition in which the individuals are nearly all homozygous. The 
assumption, therefore, here involved is that the heterozygous condition 
Aa in general is no more variable than the homozygous conditions AA 
and aa. There are many exceptions to this rule, so that it can be said 
fairly that a slight increase in variability need not be taken to invalidate 
this first condition. 

2. In all cases where the parent individuals may reasonably be pre- 
sumed to approach complete homozygosis, F2 frequency distributions 
arising from extreme variants of the Fi population, should be practically 
identical, since in this case all Fi variation should be due to external 
conditions. 

This follows because all the Fi individuals in such a case presumably 
belong to the same genotype. The student should compare this state- 
ment with the ideas developed in the chapter on pure lines. 

3. The variability of the F2 population from such crosses should be 
much greater than that of the Fi population. 

This proposition is merely a statement to the effect that segregation 
for size factors takes place in the same manner as segregation for other 
factors. Accordingly in F2 a series of forms is obtained depending upon 
the recombination of size factors. Furthermore, it may be stated that 
with a given range, the less the number of size factors involved the 
greater will be the increase in variability in F2. The maximum vari- 
ability as measured by the coefficient of variability would be attained 
by a single factor difference. As the number of factors for the given 
range increases the coefficient of variability of F2 decreases, so that with 
a very large number of factors the limiting value is that of the Fi popula- 
tion. It is therefore possible to have size differences which give inter- 
mediate forms which appear to breed true in subsequent generations 
unless a large number of individuals be grown. 

4. When a sufficient number of F2 individuals are available, the 
grandparental types should be recovered. 

Simply a restatement of the consequences of factor recombination. 



186 GENETICS IN RELATION TO AGRICULTURE 

In general the number of individuals necessary for recovery of the 
parental types depends upon the number of factors involved, so that 
with large numbers the expectations rapidly become very slight. 

5. In certain cases individuals should be produced in F2 that show 
a more extreme deviation than is found in the frequency distribution 
of either grandparent. 

This follows from a consideration of cases like the following. If 
XXAABBcc and XXaabhCC be crossed, there will be obtained by re- 
combination in F2 individuals of the genetic constitutions XXAABBCC 
and XXaahhcc which would be larger and smaller respectively than the 
grandparental types. 

6. Individuals from various points on the frequency curve of an Fi 
population should give F3 populations differing markedly in their modes 
and means. 

This of course depends on the fact that the F2 individuals represent 
a series of genotypes which give F3 populations depending on their par- 
ticular genotypic constitutions. 

7. Individuals either from the same or from different points on the 
frequency curve of an F2 population should give F^ populations of diverse 
variabilities extending from that of the original parents to that of the 
F2 population. 

The variability of a population depends on the genotype of the F2 
plant selected. If this plant be heterozygous for many factors its 
variability obviously will exceed that of one heterozygous for but few 
factors. That plants occupying the same point on a frequency curve 
may possess different genotypes and be heterozygous for differing 
numbers of factors is self-evident, and is well illustrated by Nillson- 
Ehle's case of color of grain in wheat, which has been treated in detail 
in a previous chapter. 

8. In generations succeeding the F2 the variability of any family may 
be less but never greater than the variability of the population from which 
it came. 

This proposition is to be taken with some reservation. Absolutely 
it holds only if the factors involved lie in different chromosomes. If 
there is any linkage between size factors then the proposition is valid 
only when the number of factors involved is large. In that case breaks 
in linkage occur as often in one direction as in the other and so would not 
greatly influence the result. If the number of factors be small, however, 
and some of them coupled, then breaks in linkage might profoundly affect 
the relations in recombination and thus render invalid this proposition. 

Castle's Hooded Rats. — Serious objections have been raised to the 
multiple factor hypothesis of size inheritance particularly on the basis of 
results of selection experiments. The multiple factor hypothesis depends 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 187 



-2 -I 



+ 3 +4- 



^ m 9: i4 A )■( A ii( 




Fig. 88.— Top row, a set of arbitrary grades used in the classification of hooded rats. 
Middle and bottom rows, skins of rats graded as indicated by the numerals above each 
skin. The animals +4, +43':4, +4^4, being entirely dark above, are shown in ventral 
view. {After Castle and Phillips.) 



188 GENETICS IN RELATION TO AGRICULTURE 

on the acceptance of the idea of factorial constancy, which a priori need 
not necessarily be a valid hypothesis. For it is, indeed, strange, if fac- 
tors are the complex chemical bodies so many have assumed them to be, 
that they should display such constancy in their relations with each other 
in spite of the intimate contact which exists between them in the cell. 
Castle, therefore, has pointed out that assuming that factors vary, it is 
possible to account for the phenomena exhibited in size inheritance with- 
out postulating the existence of so great a number of factor differences. 
The case in point is that of the hooded pattern in rats. During the 
progress of the experiments, over 25,000 rats have been reared and the 
color patterns studied, so that this case has been studied as extensively 
perhaps as any bearing on the subject. The rats have been graded ac- 
cording to an arbitrary scale which is designed to express the extent of 
pigmentation in such a way that the results of the experiments may be 
analyzed statistically. The set of arbitrary standards employed together 
with some rats which have been classified according to it are shown in 
Fig. 88. As a result of these investigations. Castle has drawn a number 
of conclusions of which the following seem most pertinent in this 
connection: 

1. The hooded pattern of rats behaves as a simple Mendelian char- 
acter in crosses with either the Irish pattern (white belly) or the wholly 
pigmented condition of wild rats. 

2. Though behaving as a unit, the hooded pattern fluctuates — that is, 
it is subject to plus and minus variations. 

3. Selection, plus or minus, changes the position of the mean and 
mode about which variation occurs. 

4. The results of such plus or minus selections are permanent, for 
return selection is not more effective than the original selection, and dur- 
ing return selection regression occurs away from the original mode, that 
is, toward the mode established by selection. 

5. During the progress of the original selection variability as measured 
by the standard deviation was somewhat diminished. 

6. Upon crossing the selected plus and minus races with each other, 
the variability was somewhat increased in Fi and was further increased 
in F2. The extreme condition (plus or minus) of the grandparents rarely, 
if ever, recur in this generation. Only one individual among 378 F2 
young has been recorded in a grade as extreme as either grandparent. 

On the basis of these and other facts Castle argues that we must 
recognize three types of inheritance: 

1. Typical Mendelian Inheritance. — Factors of allelomorphic char- 
acters may meet each other generation after generation in a common 
zygote, but segregate in gametogenesis without any apparent modifica- 
tion following their conjugation in the zygote. 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 189 

2. Typical Blending Inheritance. — Factors of allelomorphic characters 
blend in the heterozygote to form factors of intermediate character. 
The factors of the heterozygote are as uniform as those of either parent 
individual. 

3. Partial Blending Inheritance. — Factors of allelomorphic characters 
segregate in gametogenesis in the heterozygote, but with modification 
due to varying degrees of blending between the two allelomorphic factors. 

• The first category here set off is that of ordinary Mendelian inherit- 
ance, and includes all cases in which there is admittedly no contamination 
by association of the members of an allelomorphic pair in the same 
individual. The second category calls for an assumption of factorial 
alteration to such an extent that all the factors of the parents as such 
disappear and only a factor determining an intermediate condition re- 
mains. The case may be illustrated abstractly in this fashion. Let the 
difference between two races genetically be that between A and a. 
The races are crossed. Now according to ordinary Mendehan assump- 
tions the factors A and a will segregate in the germ cells of the offspring 
and without factorial contamination. According to the assumption of 
typical blending inheritance, however, the two factors A and a when they 
meet in the hybrid immediately interact and this interaction gives an in- 
dividual having the genetic constitution A' A' rather than Aa, the symbol 
A' representing a factor intermediate in its character expression between 
A and a. As a consequence of this change in the factors involved such 
an individual, although of hybrid origin, is genetically not a hybrid and 
would consequently breed true. Admittedly the cases which fall into 
this category are not common, and it is a matter of debate whether any 
have thus far been found. As was pointed out in connection with the 
discussion of the conditions for a Mendelian interpretation of quantita- 
tive inheritance, the same relations find a consistent Mendelian explana- 
tion in those cases in which the number of factor differences is very large. 
The third category assumes that a variety of conditions may arise as 
the result of the production of an A a individual. The blending may be de- 
finite so that A and. a become A' and a', or it may be indefinite and give 
rise to a series of factors Ai, A2, A3, . . . An and ai, a^, as, . . . an, 
all of which may be represented in the gametes of the heterozygous in- 
dividual. When definite factor contamination is assumed it may be 
regarded simply as an expression of the condition that the reacting 
system Aa reaches an equilibrium when it becomes A' a', a condition for 
which there are many analogies in chemical reactions. In case some of 
the original factors still remain, such a heterozygous individual would 
produce gametes A, A', a', and a, which by conjugation would produce a 
variety of forms in its offspring. In such a case an individual AA', or Aa' 
might give rise to the establishment of a new equilibrium, but equally 



190 GENETICS IN RELATION TO AGRICULTURE 

definite with respect to the factors involved. Such an assumption is 
obviously an hypothesis of extensive multiple allelomorphism in which the 
members of the system Ai, A2, A3, . . . An when in contact with each 
other react to form new allelomorphs. On the basis of a chemical con- 
stitution for the locus in the chromatin material, such an hypothesis would 
appear not improbable, for if the nucleus of this locus were identical in all 
the allelomorphs involving it and the changes in it were changes which took 
place around the fringe of the molecule, inside and end chains perhaps', 
then there appears to be no good reason for believing that two such similar 
allelomorphs when in intimate association with each other should not 
interact to form intermediate factors. While the very existence of the 
second and third categories is not generally accepted by geneticists, it 
must be admitted that, provided the assumptions of factorial alterability 
be accepted, they can explain the known facts of size inheritance. 

That a factor as such may vary and that selections of variations in the 
expression of such a factor may permanently alter its expression in any 
desired direction. Castle has endeavored to show in extensive selection 
studies with hooded rats. The selections v/ere made in both plus and 
minus directions and in each case selection did establish a permanent 
variation in the direction in which it was made. The condensed results 
of these selection experiments involving over 30,000 individuals are given 
in Tables XXXIV and XXXV. These results show very clearly that 
selection has definitely changed the mode around which the pattern 
fluctuates and in opposite directions in the two series. Since the hooded 
pattern has been shown to be a simple Mendelian recessive to self color, 
does this evidence prove that selection has modified the factor for hooded 
pattern? 

Those who hold to the multiple factor idea of size inheritance contend 
that it does not, and for several reasons. In the first place the difference 
between hooded and self-colored rats, while in the main due to a single 
factor difference, may at the same time involve other minor differences in 
a number of factors which influence the extent of pigmentation when the 
factor for hooded pattern is present. According to this view selection 
would result in attaining a homozygous condition for certain of these 
modifiers and, therefore, in increasing or decreasing the area of pigmen- 
tation in the direction of selection. There may be a number of such 
modifiers and others may arise from time to time by mutation. It is 
interesting to note that Castle records the appearance of two such 
mutant individuals. These mutant individuals when tested with each 
other and the forms from which they arose, displayed a type of inher- 
itance which indicated that their origin involved a single factor differ- 
ence for extent of pigmentation from the parental group. Might not 
other mutations have arisen which, on account of their lesser magnitude 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 191 



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192 GENETICS IN RELATION TO AGRICULTURE 

and intergrading with the parental forms, escaped notice as mutations 
but were selected for continuing the experiment? Such an assumption 
would render intelligible the efficacy of return selection which would be 
difficult of interpretation on even a multiple factor theory of heredity. 

That such a system may exist in qualitative characters has been 
shown by Bridges for the relation between eosin eye color and its modifiers 
in Drosophila. One modifier called dark intensifies the eosin character. 
The other six modifiers are all diluters. Thus cream a changes eosin to 
pale yellow or cream color, cream h has a similar effect, but not so marked. 
Whiting changes the eosin color to white, so that eosin-whiting flies are 
indistinguishable from white-eyed flies in color. In these cases there is 
no question as to the operation of a multiple system of factors, for the 
specific factors have arisen singly by mutation and their linkage relations 
establish completely their identities. Nevertheless taken together they 
would give in a qualitative character a remarkably close imitation of 
the behavior of Castle's hooded rats. 

If, however, we assume with Castle that factors like characters are 
variable and that allelomorphic contamination occurs, then we may offer 
an explanation based on a consideration of a single allelomorphic system. 
For such an explanation the hooded pattern may in general be represented 
by h, and its dominant allelomorph, the fully colored condition, by H. 
Self-color is dominant to hooded, but the hooded condition varies greatly 
in the amount of pigmentation present in the coat. These variations 
appear to be correlated with definite factor variations, consequently wc 
may designate the factors determining the various degrees of pigmen- 
tation in the hooded pattern by hi, h^, hs, hi, . . . hn. This series runs 
from individuals which show practically no color to those which display 
almost a self-colored coat. If we assume that the character expression 
of an animal of the genetic constitution hihw be intermediate between 
that of an animal of the genetic contitution hihi, a very light type, and 
one of the constitution hiohio, a very dark type, then we may point out 
what would occur if selection were carried out in the progeny of such 
an individual. In the fii'St place the genetic constitution hihio of such an 
animal represents merely the values of the gametes that united to form 
the zygote. They are assumed to interact immediately, so that perhaps, 
in addition to the factors hi and hio, such a zygote will produce gametes 
bearing for the most part the factors h^ and he, representing a sort of 
equilibrium for the interaction of the factors hi and hio- There would, 
therefore, be in the progeny of such an individual some individuals of 
the genetic constitution hih^ which would be lighter than the parents, and 
some of the genetic constitution h^hio which would be considerably darker 
than the parents. If other products of this reaction, such as hs, h^, /17, hg, 
etc., were also produced, and like the original reacting factors hi and hjo 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 193 

occurred in relatively infrequent numbers in the gametes, then other 
combinations would result. A graded series running from light to dark 
would then be produced, but since the mid-products, h^ and ha, would be 
by far most numerous, in small progenies most of the individuals would 
display a mid-condition of pigment development. On such an explana- 
tion any particular type of hooded pattern would be allelomorphic to the 
wholly pigmented condition or to the Irish condition, if these latter two 
with hooded be members of a system of triple allelomorphs. Also, the 
variability of the factor due to interaction with whichever other member 
of the allelomorphic pair it happened to be in contact would account 
for the variability in the expression of the hooded pattern following 
hybridization. 

The progress of selection in such a form on the basis of a single 
factor as determining not only the hooded pattern, but also the extent of 
pigmentation in the hooded condition requires us to assume an instability 
in the factor even when in the pure condition. We would, therefore, 
assume that, in an animal of the constitution h^h^ with respect to the 
factor for hooded pattern, the instability of the factor leads at times to 
the production of gametes by such an animal bearing the factor h4 
on the one hand, or h^ on the other hand. If such gametes were produced 
relatively infrequently, they would almost invariably mate with 
gametes of the genetic constitution h^. The resulting progeny would 
have the genetic constitutions /14/15 and hohe and they would be slightly 
lighter and slightly darker respectively than the bulk of the animals 
of the genetic constitution /15/15. Selection of such individuals would 
rapidly lead to the production of races of the genetic constitutions hJiA 
and h^he. Individuals of the genetic constitution ^4/14 on account of the 
variability in the factor itself would produce some gametes bearing the 
factor hs or the factor h^, and by continuing the same process of selection 
a still lighter race of the genetic constitution hshs might soon be estab- 
lished. Assuming, therefore, that factor variability of this type occurs it 
is not difficult to see how a continuous process of selection such as Castle 
has employed should finally result in the establishment of new races 
differing markedly in their character expressions and possessing a different 
but related genotype to that of the original type from which selection has 
been made. Moreover, such an hypothesis accounts for the observed fact 
that return selection is just as efficient but no more so than the original 
selection in changing the mean of the races, a fact which presents some 
difficulties for a strict multiple factor interpretation. 

It should be stated that this hypothesis of factor variability does 
no violence to our conception of the nature of factors, except with respect 
to a rather ill-established belief in factor constancy. The continuous 
change in a factor such as we have outlined above reminds us very forcibly 

13 



194 GENETICS IN RELATION TO AGRICULTURE 

of the behavior of certain chemical systems. It is a well-known fact 
for instance that in some systems an equilibrium is reached when a certain 
proportion of two substances are present in a chemical system. Thus a 
system consisting of A and B, two compounds mutually convertible 
into each other, may reach an equilibrium when say 2 parts of A and 1 
of B are present in the system. If now a certain proportion of A is 
removed from the system, enough of B will be converted into A to rees- 
tablish the old equilibrium of 2A : IB. It is not difficult to see, there- 
fore, that continuous removal of A from such a system would finally 
result in the conversion of all of B into A. Assuming, therefore, that 
our original system consisted merely of an unstable chemical compound, 
it might be possible by continuously removing a certain product of its 
instability to gradually alter the system in a given direction, much as we 
have outlined the case for alteration of the hooded pattern by continuous 
selection in rats. Since such changes are usually reversible, the efficacy 
of return selection is adequately accounted for. 

Nevertheless, although it must be admitted that an interpretation 
such as we have given above may account for all the known facts of 
quantitative inheritance, and as the student can readily see it may be 
employed to interpret the entire set of eight conditions which East 
has outlined, we advocate the strict multiple factor hypothesis of size 
inheritance for the following reasons: 

1. It is definitely known that large numbers of loci may be concerned 
in the expression of a certain character. Morgan has stated that over 
twenty-five factors are known to be concerned with eye color in Droso- 
phila, and similarly a large number of factors affect body color and wing 
characters. The assumption of large numbers of factors as concerned 
with a single character does not, therefore, do violence to modern con- 
ceptions of factor and character relationships. 

2. Size is a complex character depending on the cooperation and 
coordination of many organs, tissues, and physiological processes. 
Some factors may, therefore, affect one organ, some another, so from this 
viewpoint a large number of factor differences might be expected to be 
present in cases of quantitative inheritance. 

3. Although factor constancy cannot yet be considered a universally 
established fact, those definite investigations which have been reported 
indicate that factors possess on the whole a high degree of stability. 
More definite work is needed along this line; provisionally it appears 
wise to consider factors for all practical purposes as constant.^ 

4. Simple factor differences are known to give size differences, 

^ That factors are relatively stable entities is being evidenced more clearly all the 
time. Witness the definite arguments advanced by Bridges and Muller respectively 
in their recent papers on "Deficiency" and "An Oenothera-like case in Drosophila." 



FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 195 

depending many times upon some definite character change in the race, 
for example, bush and Cupid sweet peas. It appears reasonable to 
refer more complex size differences merely to differences in several such 
definite characters. 

5. Factor systems affecting a single character have been worked out 
definitely, which in the complexity of the interrelations they display, 
rival those interpretations which have been postulated for cases of 
quantitative inheritance. The trend of investigation seems to establish 
more firmly all the time the probable validity of the multiple factor 
interpretation of quantitative inheritance. 



CHAPTER XI 

INHERITANCE OF SEX AND RELATED PHENOMENA 

In the description of the chromosome relations obtaining in the distri- 
bution of hereditary units, we have had occasion to show how sex in one 
form, Drosophila a7npelophila, depends upon differences in the chromo- 
some constitution. In this species three pairs of chromosomes have equal 
members in both sexes, but the remaining pair in the female consists of 
two equivalent X-chromosomes, in the male of one X-chromosome like 
those in the female paired to an unequal F-chromosome. The distri- 
bution of sex-linked factors finds a logical explanation in their location 
in the X-chromosomes, and in Drosophila more than fifty sex-linked 
factors have been studied. But thus far the F-chromosome has not been 
demonstrated to carry any of those factors which are known to be located 
in the X-chromosome. When the chromosome relations obtaining in 
the inheritance of sex in Drosophila are outlined they are found to be 
as follows: 

XX X XF 




XX 



Morgan has called this the XF type of sex inheritance. This type 
of sex inheritance is characterized by the fact that females are homozy- 
gous for the sex determiners and males are heterozygous for them. 
Accordingly females produce but one kind of egg with respect to the sex 
determiners borne by them, but the males produce two kinds of sperm 
in approximately equal numbers. These two kinds of sperm have been 
called female-producing and male-producing sperm, because normally 
when a female-producing sperm fertilizes an egg a female is produced and 
when a male-producing sperm fertilizes an egg a male is produced. 
The production of male and female producing sperm in approximately 
equal numbers and random mating with the egg cells accounts for the 
approximate equality of the sexes in each generation. 

The XF type of sex inheritance is characteristic of a large number of 
forms. Apparently all mammals, including man, belong to this type, 
a number of insects, and the plants Bryonia and Lychnis. The evidence 
in some cases is based on the results of sex-linked experiments, in some 

196 



1 



INHERITANCE OF SEX AND RELATED PHENOMENA 197 

cases on favorable cytological evidence, but in only a few cases has 
satisfactory evidence been secured from both sources. 

In a previous chapter we have shown in detail how a sex-linked char- 
acter in Drosophila is inherited. By referring to the list of factors in 
Drosophila it may be seen that in this insect about fifty factors are known 
to belong to the first chromosome, and, therefore, to display the sex- 
linked type of inheritance. Although cases of sex-linked inheritance 
are known in' other animals, in none do we have as complete a body of 
knowledge as in Drosophila. Nevertheless, there is a sufficiency of 
other cases to lend strong support to the evidence derived from the Dro- 
sophila investigations. In man particularly several sex-linked factors 
are known, and the evidence in support of this analysis is fairly 
satisfactory. 

A typical case in man is that of color-blindness, which is much more 
common in males than in females. The factor for color-blindness may 
be called b and its normal allelomorph B. A normal-visioned woman is 
then of the genetic constitution (BX)(BX), and a normal man is (BX)Y. 
The corresponding abnormal forms are for women (bX) (bX) and for men 
(bX) Y. Since the factor for color-blindness is recessive, a woman of the 
genetic constitution (BX)(bX) will have normal color vision. In this we 
see the reason for the greater number of men that are color-blind. A man 
with a simplex dose of the factor is color-blind, because the F-chromosome 
as in Drosophila carries no demonstrable factors. In the simplex woman, 
(BX)(bX), on the other hand, the dominant allelomorph determines the 
type of color vision, so that a normal woman is produced. Simplex 
women are just as common as simplex men, the greater number of men 
displaying the color blind character is simply due to the different chromo- 
some constitutions of the two sexes. 

The relations which exist in the inheritance of color-blindness are 
exactly the same as those which exist in the inheritance of white eye color 
in Drosophila. A normal woman {BX){BX) mated to a color-blind 
man {bX) Y produces in Fi normal daughters of the genetic constitution 
{BX) {bX) and normal sons of the genetic constitution {BX) Y. These Fi 
normal sons are of exactly the same genetic constitution as all other nor- 
mal men and, therefore, although they had a color-blind father, they 
can never transmit the defect. The normal Fi women of the genetic 
constitution {BX){bX)^ however, when mated to normal men produce 
daughters of the formulae {BX){BX) and {BX){bX), all of which are, 
therefore, normal, and sons in equal numbers of the constitution {BX) Y, 
normal, and {bX)Y, color-blind. A simplex woman, therefore, although 
she does not herself exhibit the color-blind character, when mated to a 
normal man, transmits that character to none of her daughters, but to 
half of her sons. A color-blind woman can be produced by the rare 



198 



GENETICS IN RELATION TO AGRICULTURE 



mating, simplex woman {BX){hX) by color-blind man Q}X)Y, or by the 
still less frequent mating of color-blind woman (6X) (bX) by color-blind 
man (bX) Y, in which latter case all the offspring whether sons or daughters 
are color-blind. A considerable list of other sex-linked factors demon- 
strate beyond question that the inheritance of sex and the distribution of 
sex-linked factors in man is strictly analogous to that which we have 
found to obtain in Drosophila. 

Non-disjunction in Drosophila. — Of particular interest from the 
standpoint of the inheritance of sex and of the relation between factors 

and the chromosomes are the results which 
Bridges has obtained from his extensive in- 
vestigations of non-disjunction in Drosophila. 
The investigations on non-disjunction had 
their origin in certain ''exceptions" which ap- 
peared from time to time in cultures of 
Drosophila. Ordinarily in the case of sex- 
linked characters when a female with the 
recessive character is mated to a male with 
the dominant character all the females in Fi 
exhibit the dominant sex-linked character 
and all the males the recessive character. 
The reason for this fact has been explained 
already, but it will be clearly apparent from 
a consideration of Fig. 89, which is a diagram 
of the results of crosses between vermilion 
females and red males. The vermilion factor 
V is borne by the sex chromosomes, and since 
the males from crosses between vermilion 
females and red males receive their only 
X-chromosome from the mother they should 
all be vermilion-eyed. The females from 
such a cross receive from the father an X-chromosome bearing the 
dominant allelomorph of v, consequently they should all be red-eyed. 
In the great majority of cases, this is the result actually obtained from 
such matings, but occasionally, about once in 1700 individuals, an 
exception, a vermilion female or a red male, is produced. The in- 
vestigation of the "exceptional" females from sych matings has pro- 
vided unique evidence in support of the chromosome theory of heredity 
and in regard to the relations existing between the sex chromosomes 
and sex differentiation. 

The production of exceptional individuals from matings such as we 
have considered above apparently results from occasional aberrant re- 
duction divisions in the female such that the two X-chromosomes fail 




Fig. 89. — The relations of the 
sex chromsomes to sex produc- 
tion and to the inheritance of 
the recessive sex-linked char- 
acter, vermilion eye color, in 
Drosophila. The straight chro- 
mosomes are the X-chromo- 
somes, and the crooked ones the 
y-chromosomes. (Adapted 
from Bridges.) 



INHERITANCE OF SEX AND RELATED PHENOMENA 



199 



to disjoin from each other. As a result eggs are occasionally produced 
which contain two X-chromosomes instead of one as is normally the case. 
In Fig, 90 are illustrated in diagram the consequences of such aberrant 
reduction divisions in the female. If the X-chromosomes fail to disjoin 
in the reduction divisions, they may be included in the egg, in which 
case an egg wjth two X-chromosomes is produced, or they may both be 
thrown out into the polar body, in which case an egg with no X-chromo- 
some is produced. This phenomenon Bridges calls primary non-dis- 
junction. An egg (vX){vX) fertihzed by a F sperm gives a (vX)(vX)Y 
zygote, and it develops into an exceptional vermilion female. An 




Fig. 90. — Diagram of the production of exceptional individuals, vermilion females and 
red males, through primary non-disjunction from matings of vermilion female by red male. 
(Adapted from Bridges.) 

egg (one which contains no sex chromosome) fertilized by a (FX) sperm 
gives a {VX)0 zygote, and it develops into an exceptional red male. 
Zygotes of the constitution ( FX) (vX) (vX) and YO are, also, possible as 
a consequence of such non-disjunction but it is certain that they die, 
consequently nothing definite can be determined as to their characters. 
The proof that non-disjunction is. the correct interpretation of these 
exceptional cases in the transmission of sex-linked characters has been 
established by breeding tests and by actual cytological examination of 
exceptional individuals. 

Assuming that homologous chromosomes pair in synapsis, in an XXY 
exceptional female two types of reduction divisions are possible. If 
the two X-chromosomes pair, then in reduction they disjoin and one 
goes^to each pole. The free F-chromosome then passes as often to 
one pole as to the other, and as a consequence, two kinds of eggs, XF 
and X, are produced in equal numbers. On the other hand, when the 



200 



GENETICS IN RELATION TO AGRICULTURE 



Vermilion XXY Female 



XY Synapsis 16% 



XX Synapsis 84'%7 




Dies(l) 



Wild Type Male (2) Wild Type Female (3) Wild Type Female (4) 
Exception 



Fertilization 
by Y Sperm 
of Wild MaW 





Vermilion Female ( 6 ) 
Exception 



Dies (6 ) 



Vermilion Male (7) Vermilion Male (8) 



Fig. 91. — Secondary non-disjunction in the female. Diagram showing the constitu- 
tion of an exceptional vermilion female, the two types of synapsis, reduction, and the 
four classes of eggs produced. Each kind of egg may be fertilized by either of the two 
(X and F) kinds of sperm of the wild male, giving the eight classes of zygotes shown. 
(After Bridges.) 



INHERITANCE OF SEX AND RELATED PHENOMENA 201 

} ''-chromosome pairs with an X-chromosome, the free X-chromosome 
then goes as often to one pole as the other and this results in the pro- 
duction of equal numbers of A", XX, XY, and Y eggs. This set of re- 
lations is shown in diagram in Fig. 91, which illustrates the phenomena 
exhibited in the production of gametes by a vermilion non-disjunctional 
female. From experimental evidence it has been determined that 
homosynapsis, 'i.e., pairing of the two X-chromosomes, takes place in 
84 per cent, of cases in non-disjunctional females and heterosynapsis, 
pairing of an X"- with a F-chromosome, in 16 per cent, of cases. A non- 
disjunctional female, therefore, will produce four types of eggs in the 
following proportions 

4 (vX) (vX) : 4 F : 46 (vX) : 46 (yX) F. 

When a vermilion non-disjunctional female is mated to a red male, the 
Fi consists of about 46 per cent, each of red females and vermilion males 
and about 2 per cent, each of further exceptions, vermilion females and 
red males. Non-disjunctional females are, therefore, characterized by 
the production of further exceptional offspring to the extent of about 
4 per cent. This type of non-disjunction consequent upon the presence 
of an extra F-chromosome is styled secondary non-disjunction. Two 
additional types of zygotes are produced as a result of secondary non- 
disjunction, those of the constitution YY which die, and those of the 
constitution Xy^Y, which make up half of the males and are not ex- 
ceptional with respect to their characters but which can transmit non- 
disjunction to a certain proportion of their offspring. It will also be 
noted that of the regular daughters half are of the constitution XX F. 
They possess the power of producing exceptions on account of the 
presence of the extra F-chromosome, but they can only be distinguished 
from their normal sisters by breeding tests or less conveniently by 
cytological examination. It is evident that an Fi population such as 
this from the mating of a vermilion female to a red male is very different 
from that which is normally obtained. 

Bridges has followed out very skilfully many of the consequences of 
the assumption that these exceptional individuals are actually due to 
non-disjunction of the sex-chromosomes and consequent production of 
various types of abnormal chromosome constitution. Thus if we con- 
sider the exceptions produced by a non-disjunctional female, it is clear 
that they are a consequence of heterosynapsis in the female. Now 
when the X-chromosome pairs with a I'-chromosome in synapsis, it 
very evidently has no opportunity to exchange chromatin material 
with the free A^-chromosome. Accordingly all the XX eggs and con- 
sequently all the exceptional daughters from such a female will belong 
to non-cross-over classes. A consideration of an actual experiment 



202 GENETICS IN RELATION TO AGRICULTURE 

will make this matter clearer. Bridges took non-disjunctional females 
known from the type of mating involved in their production to be of 
the genetic constitution (WVFb'X)(w''vfb'X)Y and mated them to bar- 
eyed males {WVFB'X)Y. Obviously the regular daughters of such a 
mating will be bar-eyed, because they receive from the father an X- 
chromosome bearing the dominant factor for bar eyes, but the excep- 
tional daughters will not be bar-eyed since both their ^-.chromosomes 
are derived from the mother. The question concerning these excep- 
tional daughters is as to whether they are invariably of the genetic 
constitution (WVFb'X)(w''vfb'X)Y or whether they may occasionally 
be cross-overs, for example (WVfb'X){w^vFb'X)Y or {Wvfb'X)(w'VFb'- 
X)Y. Since the loci involved in this case are W = 1.1, V = 33.0, 
and F = 56.5, normal crossing-over should give about 50 per cent, of 
cross-overs. By testing the exceptional females again with bar males 
of the above genetic constitution, the distribution of the males into 
phenotypes serves as an accurate indication of the genetic constitution 
of the mother. In every case in tests of thirty-seven exceptional daugh- 
ters, wild type males (WVFb'X)Y and eosin vermilion forked males 
(w^vfb'X) made up the largest classes. This indicated that the females 
were all of the genetic constitution (WVFb'X){w''vfb'X)Y, and, there- 
fore, were non-cross-overs. 

The above facts are to be taken in conjunction with the fact that 
crossing- over actually may occur in non-disjunctional females in homo- 
synapsis. We have pointed out in another place that crossing-over 
does not occur in males. Now in non-disjunctional females the occur- 
rence of heterosynapsis might well set up a condition like that which is 
responsible for non-crossing-over in the male for we would have duplicated 
the exact type of reductional divisions which occur in the male aside 
from the presence of an unpaired X-chromosome in the reduction spindle. 
But as a matter of fact the presence of the F-chromosome does not appear 
to affect crossing-over between the X-chromosomes in homosynapsis. 
Thus Bridges has summarized the data for crossing-over in non-dis- 
junctional XX Y cultures and compared them with the data for crossing- 
over in normal XX cultures with the results given in Table XXXVI. 
Far from resulting in no crossing-over the presence of the F-chromosome 
actually appears to have increased the per cent, of crossing-over be- 
tween loci in the X-chromosomes. No reason can be readily assigned 
for this increase in crossing-over, but it is of interest to note that the 
presence of a F-chromosome does not preclude the occurrence of cross- 
ing-over. 

In Fig. 91 it is shown that half of the regular sons of a non-dis- 
junctional female are of the type XYY instead of XF as normally. The 
hereditary behavior of such males as determined by experiment is shown 



INHERITANCE OF SEX AND RELATED PHENOMENA 



203 



Table XXXVI. — A Comparison of Cross-over Values from Normal and Non- 
DxsjUNCTiONAL CULTURES IN Drosophila {Data from Bridges) 





XX cultures 


XX y cultures 


Increase 




Loci 


Total 


Cross-over 
value 


Total 


Cross-over 
value 


increase 


WT 


2,600 
15,177 
6,262 
1,699 
2,600 
6,262 


24.4 
29.5 
43.1 
43.6 
5.6 
22.4 


2,436 
12,817 
3,651 
257 
2,436 
3,651 


26.0 
33.7 
49.8 
53.0 
5.9 
26.0 


1.6 

4.2 
6.7 
9.4 
0.3 
4.4 


6.6 


WV 


14.2 


WF 


15 5 


WB' 


21.6 


TV 


4.6 


VF 


19.6 













Vermilion XYY Male 



XY Synapsis 67% 



Reduction! 



YY Synapsis 33% 



Sperm 




Offspring! 

Sable Male (1) Wild Type Female (2) Wild Type Female (3) Sable Male (4) 

Fig. 92. — Diagram of secondary non disjunction in the male. Four kinds of sperm 
are produced, but none of these lead to the production of phenotypic exceptions in Fi. 
(.After Bridges.) 



204 GENETICS IN RELATION TO AGRICULTURE 

in diagram in Fig. 92. There are two possible types of synapsis in 
non-disjunctional males, the ordinary type of heterosynapsis in the male 
in which Y is paired with X, in which case one F is free, or the YY type 
of homosynapsis in which the X-chromosome is free. Obviously, if 
these two forms of synapsis take place according to the laws of chance 
homosynapsis will occur twice as often as heterosynapsis. Assuming 
this to be true the gametic series of a non-disjunctional vermilion male 
will be as follows: 

2(vX)Y:2Y:l{vX):lYY. 

When such males are mated to sable females, all the males in Fi are sable 
and all the females are of the wild type. No exceptions, therefore, are 
produced in Fi, but two-thirds of the daughters are non-disjunctional 
and should give exceptions in F2. Bridges showed that among fifty- 
four females only fifteen gave no exceptions in F2. Consequently 72 
per cent, of the females must have been non-disjunctional, and this 
may be regarded as an insignificant deviation from the expected value of 
67 per cent. 

We cannot go into detail concerning any other of the numerous 
points which have been investigated with respect to non-disjunction and 
its attendant phenomena. That non-disjunction is not due to the pres- 
ence of a sex-linked factor was proven by two lines of experimental 
evidence. In the first place such a factor should have shown linkage 
relations with the sex-linked factors and consequent crossing-over in 
definite percentages with different loci. An extensive series of matings 
showed, however, that non-disjunction was entirely independent of 
linkage relations. The other line of evidence related to attempts to 
establish pure stock of non-disjunction. These attempts failed com- 
pletely, a fact readily explainable on the basis of non-disjunction, but 
reconciled with considerable difficulty to the factor idea. If this were 
not sufficient evidence, the results of cytological examination are cer- 
tainly conclusive. Examination of a number of exceptional females 
showed them to be of the chromosome constitution XX Y, and examina- 
tion of regular females from non-disjunctional mothers demonstrated 
that about half of them were XX Y, as was to be expected from theory. 
In brief the entire series of investigations give unique support to the 
chromosome theory of heredity, for throughout in this exceptional 
behavior of the hereditary mechanism, the factor distribution exactly 
parallels the unusual history of the X-chromosomes. 

From the standpoint of the inheritance of sex the investigations on 
non-disjunction throw interesting sidelights on the relations of chromo- 
some constitution to sex. Thus females may be of the constitutions 
XX or XX Y or even XX YY. Evidently, therefore, the presence of the 



INHERITANCE OF SEX AND RELATED PHENOMENA 205 

extra }^-chromosomc has no influence on the determination of. sex, 
although it does give rise to unusual relations in the production of 
gametes. Zygotes of the constitution XXX would presumably be 
females, but they die and consequently nothing can be determined as 
to their behavior. Males can be either normal XY or exceptional 
XYY and XO. The last, although normal males in appearance, are 
always sterile. The y-chroniosome, therefore, must play some definite, 
positive role in gametogenesis, although we are at present unable to state 
just what its function is. Along with the preceding cases of female 
constitutions, these different types of males indicate that the determina- 
tion of sex depends upon the number of X-chromosomes present. If 
two be present, a female is produced and the presence of one or two super- 
numerary F-chromosomes does not alter this fact. If only one A^'-chromo- 
some is present a male is produced, and it is immaterial whether no 
Y is present or whether one or two such chromosomes are present. 
Throughout, the inert nature of the F-chromosome is emphasized, the only 
evidence we have of its positive action being the sterility of XO males. 
It is important also to note that the derivation of the chromosomes, 
whether from the female or from the male, does not influence the sex 
of the oiTspring. Ordinarily a male is produced when a gamete from the 
female bearing an A-chromosome is fertilized by a gamete from a 
male which bears a F-chromosome. In non-disjunctional strains, 
however, some males are produced from the union of a F-bearing egg 
with an A-bearing sperm, exactly the reverse of the usual procedure. 
Also in such strains some females are produced by the union of an egg 
containing two A'-chromosomes with a F-bearing, or ordinarily 
male-producing, sperm. Non-disjunction, therefore, establishes firmly 
the intimate relation between chromosome constitution and sex 
determination. 

The WZ Type of Sex-inheritance. — A method of sex-inheritance 
exactly the reverse of the AF type is that which Morgan has styled the 
WZ type of sex-inheritance. In this type of sex inheritance the females 
are heterozygous for a sex-determiner and the males homozygous. If 
we diagram the relations which exist here, they will be as follows : 

WZ X zz 

I \ I 

w z ^-z 



WZ 



The classical example of this type of sex-inheritance is Abraxas 
grossulariata, and, as in the AF type, the evidence for the relations 
obtaining in the inheritance of sex was given by the behavior of a sex- 




206 



GENETICS IN RELATION TO AGRICULTURE 



linked character. As it occurs in the wild, the currant moth' is usually 
of the typical form which is characterized by dark markings on the wings 
which although highly variable are of characteristic shape and arranged 
in a definite pattern. This is the form styled grossulariata. Occasionally 
in nature, however, a female is discovered which is much lighter than 




Fig. 93. 



-Diagram illustrating the inheritance of laclicolor type iu Abraxas. A lacticolor 
female mated to a grossulariata male. {Adapted from Morgan.) 



the type on account of a reduction both in number and size of the black 
markings of the wings. This is the form styled lacticolor. It is of in- 
terest to note that according to Doncaster, save in one doubtful case, 
only females of the lacticolor type have been discovered in nature. 

The inheritance of lacticolor type is illustrated diagrammatically in 
Figs. 93 and 94. In these diagrams the TF-chromosome is represented 



INHERITANCE OF SEX AND RELATED PHENOMENA 207 

as containing no factors, and the Z-chromosomes, as containing either the 
recessive factor I for lacticolor, or the dominant allelomorph L which con- 
ditions the development of the grossulariata type. When lacticolor 
females from nature are mated to grossulariata males, Fi consists of 
grossulariata males and females of the genetic constitutions W(LZ) 




Fig. 94. — Diagram illustrating the inheritance of lacticolor type in Abraxas. A 
grossxdariata female mated to a lacticolor male, the reciprocal cross of that represented In 
Fig. 93. {Adapted from Morgan.) 

and (LZ){IZ) respectively. The females are genetically the same as 
those of a pure race of grossulariata. Abundant experimental evidence 
demonstrates conclusively that not only are they themselves grossulariata, 
but they cannot transmit anything but the grossulariata character to 
their offspring. The Fo consists of grossulariata males half of which are 



208 GENETICS IN RELATION TO AGRICULTURE 

homozygous for grossulariata, therefore {LZ){LZ), and half heterozygous 
{LZ){IZ). Of the females half are grossulariata W{LZ) and half lacticolor 
W{IZ). No lacticolor males are produced in this generation, but they 
may be obtained from matings of heterozygous grossulariata males (LZ) 
(IZ) with lacticolor females W{IZ). The reciprocal cross requires no 
special explanation, since it is perfectly clear from the diagram just how 
the lacticolor factor is transmitted in such cases. Throughout, the whole 
set of experimental evidence duplicates exactly the relations found to 
exist for the inheritance of white eye color in Drosophila except that the 
sex relations are reversed. 

The cytological relations in Abraxas do not appear to rest upon as 
firm a basis as those in Drosophila. Apparently there are normally 56 
chromosomes in both the male and female, and no pair are obviously 
unequal in either sex. Apparently then the TF-chromosome in the female 
is about the same size as the homologous Z-chromosome, but like the 
Y in Drosophila it is a neutral chromosome, i.e., it carries none of the 
dominant sex-linked factors. 

Some additional cytological evidence is provided by examination 
of lines giving aberrant sex ratios. Doncaster discovered certain strains 
in which some of the females gave only female offspring, others only a 
few sons, and still others the normal 1 : 1 ratio. In these strains the 
males had 56 chromosomes, but the females only 55. As Bridges points 
~ out, if 56 is the normal chromosome number for the females of Abraxas, 
then those females having 55 chromosomes may be regarded as of the 
ZO type, corresponding to the XO mates in non-disjunctional strains of 
Drosophila. Such females produce eggs some with 27 and others with 
28 chromosomes. If as Doncaster's early observations seemed to show, 
the odd chromosome ordinarily is included in the polar body, then the 
eggs would contain mostly 27 chromosomes, and these on fertilization 
would give 55 chromosome zygotes, presumably females of the ZO type. 
Later observations of Doncaster's, however, do not confirm the conclusion 
that 27 chromosome eggs are more frequent than those containing 28 
chromosomes. Moreover, although this is perhaps not a very weighty 
argument, it is not clear why ZO females in Abraxas, if such exist, should 
not be sterile like their counterparts, the XO males in Drosophila. 

It is of considerable interest that exceptions in the transmission of the 
sex-linked character lacticolor occur in Abraxas just as they do in Dro- 
sophila. The mating grossulariata female by lacticolor male should give 
only lacticolor females and grossulariata males. However, Doncaster 
found among 611 females, the offspring of 27 such matings, three gros- 
sulariata females and two of these were in the same brood. Assuming 
that the two which were in one brood represented cases of secondary 
non-disjunction, it would appear that primary non-disjunction in Abraxas 



INHERITANCE OF SEX AND RELATED PHENOMENA 209 

is not certainly more frequent than in Drosophila. There appears at 
present to be good reason for accepting the explanation of non-disjunction 
for these exceptional cases, although Doncaster has advanced the sugges- 
tion that, if the sex-differentiator be assumed to occupy a definite locus in 
the Z-chromosome, then, if the Z-chromosome divides in such a way that 
the factor / is separated from the sex factor, exceptions will be produced. 
This case has not yet been worked out as carefully as has that in Droso- 
phila, but it presents so many close analogies that the possible interpreta- 
tion is fairly clear. 

Of forms showing the WZ type of sex inheritance a number are known. 
Moths and butterflies appear to exhibit this type universally, and such 
birds as have been investigated are all of the WZ type. A familiar 
example is that displayed by the barred pattern factor in poultry. When 
black hens are mated to barred cocks, Fi consists of barred hens and 
barred cocks and F-2 of 2 barred cocks : 1 barred hen : 1 black cock. The 
reciprocal cross barred hen by black cock gives in Fi black hens and barred 
cocks, and in Fo 1 barred cock : 1 black cock : 1 barred hen : 1 black hen. 
These are the relations which Pearl and Surface have demonstrated for 
crosses between the Plymouth Rock fowl and the Cornish Indian Game. 
The relations are exactly like those in the crosses of grossulariata and 
ladicolor, to diagram them it is merely necessary to substitute barred for 
grossulariata and black for laaicolor. 

A number of other characters in birds display the WZ type of sex- 
linked inheritance. Red-eye color in canaries behaves like ladicolor in 
Abraxas when contrasted with black-eye color, but exceptions seem to be 
unusually numerous. In pigeons a number of factors are known to be 
sex-linked. Thus in turtledoves normal color is sex-linked when con- 
trasted with white; and in the domestic pigeon the factor for intense 
coloration is sex-linked. In the fowl Bateson and Punnett have shown 
that a factor for silky plumage is sex-linked, and Pearl has demonstrated 
the existence of a sex-linked factor for high egg production. The latter 
case because of its economical importance will be given full treatment in 
another place. Besides these there are many other suspected cases, 
but they all occur either in moths and butterflies or in birds. 

Finally it remains to call attention to another analogy between the 
XY and WZ types of sex-inheritance. It is a fact firmly established by 
abundant experimentation that no crossing-over takes place in the male 
of Drosophila. Not enough evidence has yet been obtained in other 
forms to indicate whether the lack of crossing-over is a general phenom- 
enon in males that display the XY type of sex-inheritance, but it is 
highly probable that such is the case. In the silkworm moth which 
may be assumed to follow the WZ type of sex-inheritance, Tanaka has 
studied very thoroughly the linkage relations exhibited by a system of 



210 GENETICS IN RELATION TO AGRICULTURE 

quadruple allelomorphs, the factors for striped, moricaud, normal, and plain 
larval pattern, and a pair of factors for yellow and white cocoon color. 
Sturtevant has pointed out that the experimental results are explicable if 
there is no crossing-over in the female, the sex-heterozygote in this case. 
Typical results are given by crosses involving striped, Pg, and plain, p, 
larval patterns and yellow, F, and white, y, cocoon colors. Thus striped 
yellow {Ps Y) {P^ Y) crossed with plain white (py) (py) gives in Fi striped 
yellow individuals of the constitution (PsY)(py) in both sexes. When 
Fi males were crossed back to plain white females, there were obtained 
2907 individuals of which 865 were striped white and plain yellow, which 
are the cross-over classes. This gives a value of 29.8 per cent, for cross- 
ing-over in the male. Similarly striped yellow males of the genetic con- 
stitution {P,y){pY) crossed back to plain^ whites gave 488 individuals 
of which 151 were striped yellow and plain white, which are the cross- 
over classes in this case. The value for crossing-over in this latter case 
is 30.9 per cent., substantially in agreement with the previous calculation. 
These results are to be compared with those obtained by crossing back 
striped yellow females of the genetic constitution {PsY){py) to plain 
white males. From such crosses 1183 offspring were reared all of which 
were either striped yellow or plain white, consequently non-cross-overs. 
In both types of sex-inheritance, therefore, no crossing-over occurs in the 
sex-heterozygote. However, in plants which have the male and female 
organs in the same individual, crossing-over takes place both in the 
formation of pollen grains and ovules. 

The relations exhibited in sex-determination in some insects are 
extremely complex and present many differences from the simple types 
which have been described above. Much painstaking cytological in- 
vestigation has been done in determining these intricate relations with 
results which for the most part confirm our general observation as to the 
essential role played by the chromosomes. One of the simplest cases is 
that of the honey bee. As is well known there are three forms of the 
honey bee; the queens, the drones, and the workers. Worker bees are 
females with their sex organs undeveloped as a result of the kind of food 
furnished them during the larval state. Queen bees lay fertilized or 
unfertilized eggs. From the former, worker bees and queen bees develop 
according as to whether they are provided with royal jelly in the larval 
stage. Unfertilized eggs on the other hand always give drones. Along 
with these observations it should be noted that under exceptional con- 
ditions worker bees lay eggs and these always develop into drones. 
From a chromosome standpoint, therefore, queen bees and worker bees 
1 This cross as reported by Tanaka actually involved moricaud larval pattern 
not plain larval pattern, but, as previously stated, it has been proved that the factor 
for moricaud occupies the same locus as the factor for plain. 



INHERITANCE OF SEX AND RELATED PHENOMENA 



211 



possess the diploid number of chromosomes and drones the haploid 
number. By moans of experimental investigations on the sex ratio 
A. F. Shull has recently shown that sex-determination in the mullein 
thrips, Anthothrips verbasci, is accomplished by the same method as in 
the honey bee, i.e., females have the diploid number and males the 
haploid number of chromosomes. 

Morgan has worked out in detail the complex type of chromosome 
relations obtaining in the inheritance of sex in the hickory phylloxeran, 

iBt Generation 
XxXx XxXx' 

stem Mother Female Proaucing Line Stem Mother-Male ProJuciug Line. 




I'olar Spindles 

of 

Stem Mother's Eggs 




2nd Generation 



XxXx 

Migrant-Female Prodocer 



XxXx 

Migrant-Male Producer 



XXxx' 




Female Egg 



Male Eggs 



XxXj 

Female 



3rd Generation 

Xx Xx' 

Male-Type I Male-Type II 

Female Producing 

Sperm 

_^^ \/ \/ 
Polar Spiudle 




Xx 

Sexual Egg 

Fig. 95. — Diagram to illustrate the chromosomal cycle of Phylloxera carycecaulis 

{After Morgan.) 

Phylloxera carycecaulis. The life cycle of this insect with respect to the 
chromosome cycles is shown in diagram in Fig. 95. There are eight 
chromosomes in this phylloxeran and of these four appear to be con- 
nected with the determination of sex. They are the only ones illustrated 
in the diagram. Beginning with the stem mothers at the top of the 
diagram, these emerge in the spring from fertilized eggs. They immedi- 
ately attach themselves to the hickory leaves, thereby causing a gall 
to be formed around them, and in this gall they lay their eggs. As 



212 GENETICS IN RELATION TO AQRICULTURE 

shown in the diagram, these eggs extrude a polar body, but the division 
is not reductional for the eggs all have four sex-chromosomes, the same 
number as the mother. These eggs hatch without fertilization into the 
winged migrant females. Of these there are two kinds, those which lay 
large eggs and those which lay small eggs, and, moreover, all which come 
from the same gall and, therefore, from the same stem mother lay the 
same kind of eggs. Accordingly the stem mothers are of two kinds with 
respect to their chromosome content as illustrated in the diagram. 
The female producing stem mothers are XxXx and the male producing 
stem mothers are XxXx', and the migrant females have the same chro- 
mosome content as the stem mother from which they were derived. 
The migrant female of the type XxXx produces large eggs which throw 
off a polar body, but do not undergo reduction. The resulting egg de- 
velops without fertilization into a minute sexual female. The other type 
of migrant females, however, lays small eggs in which, prior to extrusion 
of the polar body, the large X's and the small x's conjugate. One of each 
of these pairs then passes out into the polar body, so that two types of 
eggs are produced Xx and Xx' and these develop without fertilization 
into the minute males. In the sexual females a true reduction division 
takes place so that her single egg is of the chromosome constitution Xx. 
The males on the other hand produce sperm cells half of which are Xx 
or Xx' according to the type of male and half of which have none of the 
sex-chromosomes. Sperms of this latter type degenerate, so that only 
female producing sperm remain. When these fertilize the sexual egg 
the resulting. eggs are either XxXx or XxXx', and give rise to the corre- 
sponding type of stem mother. This completes the complicated life 
cycle in this form, and illusti'ates again the close dependence of sex- 
determination on chromosome content. 

In plants only a few cases of sex-inheritance have been studied and 
these for the most part inadequately. Two of these, namely Bryonia 
and Lychnis, appear to display the XY type of sex-inheritance, but in a 
somewhat modified form. Thus Correns crossed Bryonia alba, which 
is monoecious, with Bryonia dioica, which is dioecious. The former 
species as a rule bears male and female blossoms on the same inflorescence, 
the female above and the male below, whereas the latter species con- 
stantly bears all male or all female blossoms on the same stem. Correns 
summarizes his results under four heads as follows: 

1. Female plants of Bryonia dioica pollinated by male plants of the 
same species give approximately equal numbers of male and female plants. 

2. Female plants of Bryonia dioica pollinated by Bryonia alba give 
only female offspring. 

3. Bryonia alba pollinated by male plants of Bryonia dioica gives 
approximately equal numbers of male and female plants. 



INHERITANCE OF SEX AND RELATED PHENOMENA 213 

4. Bryonia alba self-pollinated gives only monoecious plants. 

If we assume that all the pollen grains and ovules of Bryonia alba 
are of one kind which is indicated by the fact that it breeds true to the 
monoecious condition, then there is no escape from the conclusion that 
female plants of Bryonia dioica produce only one type of ovule but male 
plants produce two types of pollen grains. Unfortunately as is often 
the case in interspecific hybrids, the Fi of this cross is sterile and con- 
sequently the analysis cannot be carried further. 

Shull, however, has studied the inheritance of sex in Lychnis dioica 
which is normally dioecious but occasionally produces hermaphroditic 
plants. Although this case has not yet been fully analyzed, the results 
thus far indicate clearly that the male is heterozygous with respect to a 
sex-determiner, and the female homozygous. The results of Shull's 
investigations may be stated under several definite heads as follows : 

1. Females with pollen from males give substantially equal numbers 
of male and female offspring. 

2. Females with pollen from genetic hermaphrodites give equal 
numbers -of hermaphrodite and female offspring. 

3. Females with pollen from somatic hermaphrodites give equal 
numbers of male and female offspring. 

4. Genetic hermaphrodites selfed give equal numbers of hermaph- 
rodite and female offspring. 

5. Genetic hermaphrodites with males give equal numbers of male 
and female offspring. 

6. Females from whatever source are genetically identical. Thus 
females from the cross female X hermaphrodite transmit the same sex- 
determiners as females from the cross female X male. 

7. In crosses between female and hermaphrodite a. small percentage 
of mutant males always appears and in crosses between female and 
male approximately the same percentage of mutant hermaphrodites appears. 

In the above resume of the experimental evidence on sex-determina- 
tion in Lychnis, the equality of sexes was only approximate, in fact females 
usually occurred in excess, and sometimes in considerable excess. 

Shull has interpreted this evidence to indicate that in Lychnis the 
hermaphroditic condition results from a modification of the male con- 
dition, and that this modification is reversible as shown by the evidence 
in 7, above. Interpreted in terms of the XY type of sex-inheritance then, 
females are XX; males, XY; and hermaphrodites, XY'; and the change 
from Y to Y' is reversible. Clearly the results indicate that males and 
hermaphrodites are heterozygous with respect to the sex-determiner, 
and females homozygous, although later investigations which have not 
yet been fully interpreted indicate that some disturbing factors are at 
work, at least in certain cases. 



214 



GENETICS IN RELATION TO AGRICULTURE 



Shull's conclusions are further supported by evidence from the 
inheritance of a sex-Hnked character in Lychnis, the only sex-linked 
character thus far known in plants. The character in question is that 
of narrow rosette leaves as distinguished from the normal broad type of 
leaf, and there are other associated character differences (Fig. 96) . The 
narrow-leaved form, called angustifolia, was discovered by Baur as a 
single male mutant individual, a significant fact when taken in connection 
with its subsequent behavior. The factors in this case are B for the 
broad-leaved condition and h for the narrow-leaved condition. Crosses 
between typica females (BX)(BX), and angustifolia males ibX)Y 




Fig. 96.^ — Adult rosettes of Lychvis dioica; on the left a plant of the normal form, typica; 
on the right a plant of the narrow-leaved form, angustifolia. {After Shull.) 

gave in Fi all broad-leaved plants (BX) (bX) females and (BX) Y males. 
Heterozygous broad-leaved females (BX)(bX) mated to broad-leaved 
males (BX)Y gave all broad-leaved females, and approximately equal 
numbers of broad-leaved and narrow-leaved males. Hermaphrodites 
were also found to behave the same way with respect to the factor B 
as did the males, which confirms the hypothetical relation supposed to 
exist between hermaphrodites and females. The evidence clearly 
indicates the existence of sex-Hnkage of the kind called for on the assump- 
tion that Lychnis exhibits the XY type of sex-inheritance. 

Secondary Sexual Characters. — Secondary sexual characters are 
those which appear as an invariable or almost invariable accompani- 
ment of a particular sex in most animal forms. They include many 
diverse things, such as the antlers in male deer, the horns of the males of 
some breeds of sheep, the mane of the lion, the power of song of many 



INHERITANCE OF SEX AND RELATED PHENOMENA 215 

birds, and various fantastic, ornamental, and combative characters, 
usually confined to the male. Much historical interest attaches to 
secondary sexual characters because of the attention directed to them 
by Darwin's theory of sexual selection. With that we have no particular 
concern in the present chapter, but shall only consider the inheritance of 
them in one form as it is related to the inheritance of sex. 

In the foregoing discussion no particular reference has been made to 
sex-factors, because after all so little is known concerning them. In 
some cases we have found sex accompanied by differences in chromosome 
content, one sex containing an equal pair of chromosomes which are 
represented in the opposite sex by an unequal pair, in another case the 
difference in sex appears to depend upon whether the individual possesses 
the haploid or diploid number of chromosomes. We have also noted 
that there are two different types of sex-inheritance, one in which the 
male is heterozygous and the other in which the female is heterozygous. 
It is only fair to conclude, therefore, that until more light is thrown 
upon these matters, the assumption that sex-determination depends 
upon a sex-factor rests on a rather slender basis. The experimental 
evidence, it is true, is strictly analogous to certain types of Mendelian 
inheritance, and an interpretation of the sex-factor may be given which 
does no violence to our ideas of the complexity of sex-differences. Thus 
it has been shown by ample evidence that the color of eyes in Drosophila 
depends upon the cooperation of a number of different factors; we can- 
not say definitely how many, but mutational changes have indicated 
that over twenty-five different loci have something to do with the re- 
actions concerned in pigment production in the eye. Yet in spite of 
this fact the presence of a single factor may make all the difference 
between a red eye and a white eye. Similarly the sex-factor may act 
in conjunction with a whole series of other factors, yet the difference 
dependent upon its presence in the homozygous or heterozygous con- 
dition may make all the difference between the two sexes. At least in 
one form, however, we have even more definite evidence of the presence 
of a definite sex-factor. Shu 11 has shown in Lychnis that where males 
are expected, hermaphrodite mutants occasionally appear. If we offer 
the same explanation for the occurrence of these mutants as we have 
offered for the occurrence of mutations in Drosophila, a change in a 
single locus in the hereditary system, then the appearance of these 
hermaphrodites might be offered as almost conclusive evidence of 
the sex-determining action of a single sex -factor in this particular case. 
The evidence here becomes even stronger when we consider the fact that 
this particular type of change is reversible. Some additional light may 
be thrown upon this question by the consideration of secondary sexual 
characters as related to the inheritance of sex, although thus far the 



216 



GENETICS IN RELATION TO AGRICULTURE 



evidence has not admitted of an entirely satisfactory interpretation. 
We shall consider one case, that which Goldschmidt has investigated 
in Lymantria as an example of the results obtained by investigations of 
this kind. 

Goldschmidt's investigations are concerned with Lymantria dispar, the 
European gypsy moth, and L. japonica, its Japanese form. As may be 
seen from Fig. 97, Lymantria is strongly sexually dimorphic, the females 
are much lighter in color and larger than the males ; japonica is somewhat 
larger than dispar, but otherwise in general agrees with it. Goldschmidt's 
investigations deal with the production of intersexual forms in crosses 




Fig. 97. — Typical forms and hybrids of Lymantria; 1 and 2, male and female of L. 
dispar; 3 and 4, male and female of L. japonica; 5-16, hybrids combining male and female 
characters. (After Goldschmidt.) 



between these two species. He has shown that with proper combinations 
of different races of these two species, intersexes may be produced which 
occupy all possible intermediate positions in a continuous series in which 
maleness and femaleness are the two extremes. Thus female intersexes, 
i.e., individuals which are of the chromosome constitution WZ, may be 
obtained which range from those that show only a very slight develop- 
ment of male characters in the feathering of the antennae to those which 
are so nearly males that they show only a faint trace of their female 
origin in a few minor characters. On the other hand, male intersexes 
of the chromosome constitution ZZ may be produced ranging from 
those which exhibit a few white flecks on the wings up to those which 



INHERITANCE OF SEX AND RELATED PHENOMENA 217 

have gone about three-fourths of the way toward the assumption of 
the entire set of female characters. 

Goldschmidt ascribes these results to differences in potency of the sex- 
factors. The European gypsy moth was found in all races to possess sex- 
factors of low potency, whereas in the Japanese races the potency was 
in general higher, but ranged from the lowest to the highest condition. 
Thus males of a moderately strong Japanese race mated to females of a 
Japanese race of slightly less potency give in Fi very low-grade female 
intersexes. When mated to a somewhat less potent Japanese race a 
higher grade of female intersexualism results, and when mated to the 
weakest European race nothing but high-grade female intersexes are 
produced. The highest grade of female intersexualism, the transforma- 
tion of those individuals which are genetically females entirely into 
males, results from matings of females of European races of the lowest 
potency to males of Japanese races of the highest potency. Now if the 
development of sexual characters depends upon the sex-factors acting 
in conjunction with other elements in the genotype, the existence of sex- 
factors or rather of systems of factors might operate in somewhat the 
following fashion. In the female the sex-factor in a heterozygous condi- 
tion acts in conjunction with a set of factors some of which are perhaps 
sex-linked, although the number of chromosomes, 62 in this case, would 
indicate that perhaps most of them were located in other chromosomes. 
In a heterozygous condition then a certain sex-factor with those factors 
with which it acts produces a female with the female set of secondary 
sexual characters. In the homozygous duplex condition the same sex- 
factor, presumably acting in conjunction with the same set of factors as 
in the female, produces a male with the male set of secondary sexual 
characters. If now there should be variations in the potency of a sex- 
factor, as Goldschmidt assumes, then a strong sex-factor, or a sex-factor 
which would interact more effectively in a given genetic environment 
would have a tendency in the heterozygous condition to throw the reac- 
tion more in the direction of that formerly conditioned by the existence 
of the normal sex-factor in the homozygous condition. Such relations 
would result in the formation of female intersexes, individuals genetically 
females so far as the chromosome constitution is concerned, but develop- 
ing male characters in a degree corresponding to the greater potency of 
the introduced sex-factor as compared with the sex-factor normal for 
the race in question. In case the introduced sex-factor, along with the 
factors with which it normally interacts and which must never be dis- 
regarded, equals in sex-determining power that of the normal sex-factor 
in the duplex condition, then we might expect to get males of the chromo- 
some constitution WZ. This appears actually to be the case in certain 
of the experiments. Similarly a weaker potency of the sex-factor might 



218 GENETICS IN RELATION TO AGRICULTURE 

be conceived to result in the production of male intersexes, i.e., individuals 
of the chromosome constitution ZZ which display female characters, 
because the weaker potency simply means a more or less close approach 
to the potency of the normal factor in the heterozygous condition and a 
consequent approach of the individual to the characters of the male. 
Goldschmidt's results are intensely interesting and promise much for 
an elucidation of the problems connected with sex-determination. 

We cannot refrain from drawing a comparison between these re- 
sults and some which have been secured in species crosses in Nicotiana. 
Thus a definite factor for calycine flower in Nicotiana tahacum causes the 
flowers to develop a petaloid calyx and a split corolla, a striking terato- 
logical form. The character is a simple recessive to the normal form in 
variety crosses but when crossed with N. sylvestris, a different species, 
the normal flower factor in N. sylvestris appears to possess a lower 
potency than that of normal flowered varieties of N. tahacum. Con- 
sequently the hybrids are intermediate with respect to the flower char- 
acter expression, all of the flowers on a given plant exhibiting some 
development of the calycine flower character. We interpret this to 
indicate that the normal flower factor of N. sylvestris does not interact 
normally with the set of factors which interact to determine the floral 
character expression in the hybrid, but that the calycine flower factor 
is able to interact normally and to its full extent with these factors. As a 
consequence the flowers of the Fi hybrid are strongly calycine. This 
interpretation is further supported by the fact which we have previously 
set forth in some detail that practically the entire set of characters are 
determined by the N. tahacum parent. It is conceivable that crosses 
with other species would show the same character of variability in po- 
tency as has been found for the sex-factors of Lymantria. At any rate 
a close analogy here exists between the behavior of sex as a character 
and the behavior of a character known to depend upon a simple factor 
difference. 

The evidence which has been presented with reference to the determi- 
nation of sex lends strong support throughout to the idea that sex-de- 
termination depends on the genotypic constitution of the individual. 
This does not, it must be clearly understood, mean that other external 
factors may not act to disturb the usual relations just as they occasion- 
ally do with other factors; but as in such cases these external factors 
must act in conjunction with the genotypic sex-factors. To assume 
that changes occur willy-nilly in the case of sex-factors is no more warrant- 
able than to assume that other factors change frequently in response to 
environmental conditions, an assumption that does violence to the high 
degree of stability which has been observed to obtain for factors in general. 



CHAPTER XII 
SPECIES HYBRIDIZATION 

In the preceding chapters an attempt has been made to show how 
character differences in a large number of plants and animals may be 
interpreted on the basis of differences in the unit factors which are dis- 
tributed to the germ cells during gametogenesis. The character differ- 
ences, however, which were analyzed, although often seemingly complex, 
were really rather simple, for rarely were more than four or five factor 
differences taken into account. In a few species of plants and animals 
the number of factors which have been investigated is considerable, but 
when compared with the number of factors which must constitute the 
entire hereditary material of a species it is an insignificant fraction of the 
total. The analyses which have been presented, therefoje, are for forms 
which possess an enormous number of factors in common. The differ- 
ences which they display are mostly unessential alterations in scattered 
loci in these systems. 

With the taxonomic question as to what constitutes a species differ- 
ence, we are not greatly concerned. It is clearly apparent that species 
as they have been named represent widely divergent differences with 
respect to the extent of separation from related species. It must be 
clear to the geneticist, therefore, that specific difference is a variable 
thing, sometimes meaning one thing, sometimes another. If we look 
at the question from the standpoint of the number of factors involved, 
we see clearly that races of plants and animals may differ in one or many 
genetic factors. Just where the line should be drawn which distinguishes 
varieties, forms, species, etc., would therefore appear to be almost wholly 
an arbitrary matter, usually to be decided from considerations of con- 
venience. Whatever it is, however, the distinction cannot well be viewed 
from the genetic standpoint, for ordinarily the systematist works with 
plants and animals which have not been investigated in such a fashion, 
and, in the case of the more widely separated forms, with those which 
cannot be so investigated. 

A genetic investigation of the difference between two species depends 
upon the possibility of crossing the species in question, and further upon 
the possibility of securing offspring from the progeny of such a cross. 
Not infrequently this latter condition is not fulfilled, for it often follows 
as a result of species hybridization that the individuals thus produced, 

219 



220 GENETICS IN RELATION TO AGRICULTURE 

although vigorous and normally developed, are totally sterile. The mule 
is a familiar example, many others could be given, but they will be con- 
sidered elsewhere along with the problem of sterility in species hybrids. 
For the present we shall consider one of the simpler cases in which the 
species hybridized, although differing very markedly in morphological 
characters, produce hybrids which appear to be fully fertile. 

Species Hybrids in Antirrhinum. — Baur crossed the wild Antirrhinum 
molle with the common garden snapdragon. Antirrhinum majus, and 
Baur and Lotsy have made extensive studies of the progenies obtained in 
successive generations of this cross and of other species hybrids in 
Antirrhinum. Antirrhinum majus and A. molle differ strikingly in a 
large number of morphological characters. The size proportions and 
general characteristics of the common snapdragon of the garden are fa- 
mihar to eVeryone. It is a strong growing erect herbaceous plant, about 
three feet high producing spikes of large zygomorphic flowers. Under 
the careful attention of commercial seedsmen it has produced a very large 
number of varieties which differ in the form and color of the flowers, in 
height and in other characteristics. Antirrhinum molle on the other 
hand is a low growing prostrate plant which is profusely branched and 
produces flowers about one-third as large as those of majus, but very like 
them in form and general appearance. The species differs from majus 
also in being apparently totally self-sterile, so that with respect to their 
genetic constitution plants of molle are normally heterozygous to some 
extent. Since 7nolle occurs in nature in a number of slightly different 
forms its self-sterility must not be lost sight of in interpreting the results 
of hybridization between it and majus. Fi of the reciprocal crosses 
molle X 7najus and majus X molle are completely self-fertile, and identical 
in every respect. Minor differences did occur but they were of such a 
nature that they could be accounted for as a result of the slight degree of 
heterozygosis of the particular plant of molle which was used as a parent. 

Baur employed a peloric majus for crossing with molle in order that he 
might follow a known factor difference throughout the investigation. 
The Fi plants in this experiment bore zygomorphic flowers, a fact which 
indicated a corresponding behavior as regards dominance for the factor 
for zygomorphic flowers in molle and majus. Six Fi plants differing 
slightly in their characters were selected as parents for the F2 generation. 
Lotsy grew the progeny of five of these, obtaining from them 624 Fo 
plants. The general conclusions which Baur and Lotsy have drawn 
from a study of these F2 plants is that the extreme range of forms dis- 
played, so great that no two plants resembled each other in all their 
characters, is a result of Mendelian segregation and recombination of 
characters; The diversity, however, was so great as to preclude the 
application of any exact factor analysis to the case. 



SPECIES HYBRIDIZATION 



221 




jPjQ 98. Flower types obtained in F- of a cross between Antinhinuin maju.-i (peloncj and 

A. molle. {After Lotsy.) 




Fig. 99. Beginning at the left, a peloric majus of the type used lacrosses with molle; 

a plant of molle; a plant much resembling molle obtained in F2; and on the extreme right 
the F3 progeny of such a plant. {After Lotsy.) 



222 GENETICS IN RELATION TO AGRICULTURE 

In one F2 population of 255 plants Lotsy was able to distinguish about 
twenty-five different flower types as shown in Fig. 98. The flower 
types were not distinct, but represented merely different steps in an 
almost continuous series, save for the discontinuity incident upon the 
sharp segregation of a group of plants which bore peloric flowers. More- 
over, within any of these flower types the plants differed greatly in a 
number of other characters, such as size, color of flower, form of leaf, 
habit of growth, etc. As regards fertility there was segregation into 
self -fertile and self-sterile plants, the former being in the majority. Of 
the 255 plants, 135 produced zygomorphic flowers, 119 peloric flowers, 
and one plant produced both zygomorphic and peloric flowers. In 
color the flowers on different plants ranged from the deep red of the majus 
parent to the pale color of molle. 

Lotsy also grew several Fs populations. One of these from an F2 
plant bearing hooded zygomorphic flowers consisted of 209 plants all 
of which were different, indicating again an extreme condition of hetero- 
zygosity. Not a single plant produced flowers displaying the hooded 
character of the parent plant. There was again a vast array of flower 
forms, twenty-three different types feeing represented. With respect 
to the peloric condition, 113 plants bore peloric flowers only, 94 zygo- 
morphic flowers, and 2 bore both types. Although several different 
colors were represented, Lotsy was able to arrange them in two classes; 
the first consisting of 153 plants approximating the red color of majus, 
and the second group of 56 plants of about the color of the pale molle 
parent. There was, therefore, a fair indication of Mendelian segrega- 
tion for color in this generation. 

In this population as shown in Fig. 99 a plant was obtained which 
very closely resembled the type of A. molle in all its characters, and re- 
produced these characters in its progeny. Other plants were obtained 
which strongly resembled majus in certain of their characters, but not so 
completely throughout. The important feature here is the fact that 
even in F2 segregation and recombination of factors have produced a 
plant which is practically identical with one of the parents. 

Other F3 populations were grown from F2 plants displaying different 
sets of characters. One of these from a zygomorphic F2 plant produced 
a population segregating for color and form (zygomorphic vs. peloric) of 
flower. Another population from a peloric F2 plant consisted entirely of 
peloric flowering plants, but in this population there were many different 
color classes. An F2 plant of the pale color of molle gave an F3 popula- 
tion consisting entirely of pale-flowering plants but showing segregation 
in form and for the peloric character. 

Obviously if results such as these are to be explained on a Mendelian 
basis, it must be assumed that a relatively large number of factor differ- 



SPECIES HYBRIDIZATION 223 

ences exist between the two species under consideration. When we 
observe the number of differences in habit, form, size, etc., which are 
known to obtain between the two species, this assumption does not appear 
to do violence to actual facts in the case. Baur has sought by systematic 
hybridization investigations to determine which of the known factors of 
the hereditary material of A. majus are also contained in that of A. molle. 
From these investigations he concludes that A. molle certainly possesses 
the factors indicated by the incomplete formula BBDDEEFFll, in which 
B represents a factor for yellow flower color; D, a factor for extension of 
pigment to the tube of the corolla; E, the factor for zygomorphic flowers; 
F, a base factor for red flower coloration which is epistatic to B; and I, 
a recessive factor which determines a low intensity of flower coloration. 
His success in determining the presence of these factors in the hereditary 
material of A. molle has led Baur to conclude that it is entirely within the 
range of possibility to analyze completely the differences which exist 
between these two undoubted species. All the unusual flower forms, 
therefore, which are obtained by crossing them are to be regarded as the 
results of peculiar factor interactions. We have pointed out in previous 
chapters that it is not always possifcle to predict the character expression 
of a given set of factors from a knowledge of their known expression in 
certain combinations. That this condition is here operative is borne out 
in part by the fact that certain flower types which appeared in F2 did 
not reappear even among fairly large numbers in the F3 generation from 
such F2 plants. We consequently can state with assurance in spite of 
unsatisfactory ratios and peculiar character expressions that the results 
obtained in this species cross may reasonably be interpreted in harmony 
with Mendelian doctrine. 

Detlefsen's Cavy Hybrids.^ — ^A similar line of investigation in animals 
has led Detlefsen to similar conclusions. He crossed the tame guinea- 
pig, Cavia porcellus, of which many different races have been produced 
under domestication, with the wild C. rufescens. The latter differs from 
the tame guinea-pig in a number of respects. It is very much smaller, 
weighing about half as much as the tame guinea-pig, and in skeletal 
measurements and other characters it is definitely set off as a distinct 
species from C. porcellus. In color it is of the agouti type common to 
all wild rodents, but the agouti differed from that of the tame guinea- 
pig in having less power to exclude black and brown from the hair than 
has the agouti of the tame animals, consequently individuals of the wild 
C. rufescens have darker coats than those of the tame porcellus. 

By crossing C. rufescens and its hybrids with porcellus with various 
races of porcellus, Detlefsen was able to study the inheritance of the 
following factors in this species cross : 

A — the agouti factor, which operates by restricting the black or brown 



224 GENETICS IN RELATION TO AGRICULTURE 

pigment in the hairs thus producing the gray or agouti pattern. There 
are variations in the regional distribution of the restrictive action. The 
allelomorphic condition a gives self-colored individuals. 

B — the factor for black. The allelomorph h conditions a brown col- 
oration instead of black. 

C — the basic color factor in rodents. The allelomorphic condition 
represented by c gives albinos. 

E — a factor conditioning the extended type of pigmentation of self- 
black or brown animals. The allelomorph e gives the black-eyed or 
brown-eyed red or yellow coat. 

R — the factor for rough or rosetted coat, as distinguished from the 
smooth coat determined by the allelomorph r. 

The work of a host of investigators has demonstrated beyond question 
the Mendelian inheritance of these factors in races of the tame guinea- 
pig. Castle in particular has demonstrated how these factors behave in 
Mendelian fashion, one among the first investigations estabhshing the 
general validity of Mendelian principles. Moreover, these conclusions 
have been abundantly confirmed by investigations with other rodents, 
which appear to possess a closely analogous series of color factors. 

Detlef sen's experiments were conducted by crossing tame female 
guinea-pigs to wild males, and then mating back the hybrid females to 
tame male guinea-pigs. This was necessary because the male hybrids 
were sterile until back crosses to the tame guinea-pigs had been made for 
two or three generations. Crossing back to the wild species was impos- 
sible on account of the scarcity of wild animals and their failure to breed 
freely under domestication. The investigations were carried through 
eight generations, during which many types of matings were made, 
and a total of 1160 hybrids were reared and studied. 

As a result of these investigations Detlefsen concludes that the wild 
rufescens is of the constitution AABBCCEErr with respect to the factors 
noted above. Moreover, the relation of these factors as respects domi- 
nance and segregation was throughout identical with the relations 
displayed in intervarietal crosses in the tame guinea-pig. Recombinations 
of factors occurred in the normal fashion so that it was possible to secure 
hybrids showing any type of coloration found in the tame guinea-pig. 
The conclusion, therefore, that interspecific crosses between C. porcellus 
and C. rufescens display complex Mendelian inheritance appears to be 
established by these investigations. 

It may be pertinent, however, to enquire whether homologous factors 
normal for the two species are really identical. If we assume that the two 
species possess similar genetic constitutions, i.e., have similar sets of 
chromosomes bearing the factors in like arrangement, it is entirely con- 
ceivable that, although the formal arrangement of factors in the heredi- 



SPECIES HYBRIDIZATION 225 

tary material might be the same for the two species, the actual factors them- 
selves might differ in certain respects, for example in the exact type of 
character expression and in their power to react with a given set of 
factors. If the differences be relatively slight, the factors might still be 
able to interact with each other approximately in the normal fashion, 
and to display allelomorphic relations dependent upon their position in 
the hereditary material. On this point Detlefsen contributes very 
important data which we shall consider somewhat in detail. 

The first set of observations relates to the differences between the 
agouti factors of C. porcellus and C. rufescens. It is a common obser- 
vation that the agouti pattern in rodents in general is a variable one. 
Some of this variability is unquestionably due to the presence of modi- 
fjdng factors, but not all such variations can be interpreted in this fashion. 
Elsewhere we have pointed out that in mice a system of quadruple 
allelomorphs includes the factors for yellow, black, gray, and gray with 
white belly. In the rabbit, Punnett's results may be interpreted as 
establishing the existence of a triple system of multiple allelomorphs 
consisting of the factors for yellow, agouti, and black. Similarly in 
the tame guinea-pig there are apparently allelomorphic variations which 
affect the agouti pattern, but Detlefsen finds, nevertheless, that these 
never condition the type of agouti presented in C. rufescens. Detlefsen 
points out that agoutis in common restrict black or brown in the sub-apical 
band of individual hairs so that the dorsal hairs present a barred appear- 
ance. More powerful restriction is shown in the hairs of the belly, but 
there is always a close correlation between the amount of restriction in 
dorsal and ventral regions, for the darker the dorsal region, the darker 
is the pigmentation of the ventral surface. The wild agouti factor was 
distinguished by its weak restricting power, so that ordinarily the yellow 
sub-apical band in the hairs of these animals was distinctly narrower than 
in some agouti guinea-pigs. In some cases the lack of restriction was so 
marked that only a slight sprinkling of agouti hairs in the adult gave 
evidence of the existence of the agouti factor. Moreover, in some cases 
the wild agouti pattern carried with it a ticked belly, a condition appar- 
ently unknown in the tame guinea-pig. Some variation was observed 
in the agouti patterns of the original individuals of C. rufescens and this 
must not be forgotten in interpreting Detlefsen's results. Dark agoutis 
produced by constantly mating wild agouti hybrids to tame non-agouti 
guinea-pigs, were mated to tame agouti animals. We may represent 
the factor for wild agouti by A', that for tame agouti by A, and that for 
the allelomorphic condition in the tame guinea-pig by a. Following this 
formula, then those dark agoutis produced by mating wild hybrid 
agoutis to tame non-agoutis must have been of the genetic constitution 
A' a. When such animals are mated to tame agoutis two types of animals 

15 



226 GENETICS IN RELATION TO AGRICULTURE 

are produced, those of the genetic constitutions AA' and Aa respectively. 
Phenotypically these two classes of individuals are exactly alike for the 
powerful tame agouti factor is alike dominant to the wild agouti factor 
A' and to the tame non-agouti factor a. When the individuals of this 
population were bred to tame non-agouti animals of the genetic consti- 
tution aa the existence of the two above-mentioned genotypes was clearly 
demonstrated, for half the individuals gave progenies exhibiting sharp 
segregation into light tame agoutis and dark wild agoutis in approximately 
equal numbers and the other half gave progenies consisting of approxi- 
mately equal numbers of light tame agoutis and non-agoutis. Other 
tests satisfactorily supported this analysis so that it may be concluded 
that the agouti factor of the wild C. rufescens is different from the agouti 
factor of the tame C. porcellus, but that they are allelomorphic to each 
other. If we consistently follow up the hypothesis which we have devel- 
oped as to the constitution of the hereditary material and the operation 
of the chromosome mechanism, this can only mean that the factors for 
agouti, although different, occupy corresponding loci in the hereditary 
system of these two species. Aside from certain observations indicating 
differences between the rough factor of wild and tame guinea-pigs we 
have no evidence as to whether or not those other factors, the inheritance 
of which was investigated, are different, but we may safely conclude 
that factors of corresponding behavior occurred at exactly the same loci 
in the hereditary system of the wild C. rufescens. 

Evidence as to the difference between the two agouti factors is also 
provided by the irregular behavior of the wild agouti factor in the hybrids. 
Although the first hybrids between the wild agouti and tame non-agouti 
guinea-pigs are mostly of the dark wild type with ticked bellies in sub- 
sequent generations there appear agoutis which are so light as to approach 
closely the light agouti type of the tame parent and others are so dark 
that the individuals show only a slight sprinkling of agouti hairs. Indi- 
viduals displaying such modifications of the wild agouti pattern show no 
very regular type of behavior, for dark individuals sometimes produce 
some light individuals and the light individuals sometimes produce 
some dark ones. The dark modification, however, is most common 
and often becomes more pronounced upon successive dilutions with 
tame blood. An interpretation of such phenomena cannot be made 
satisfactorily unless we consider the agouti factor as a member of a com- 
plex system of factors which together operate to give the agouti type 
of coloration. From this standpoint it is not at all strange that the 
wild agouti factor acting in conjunction with a corresponding system 
of factors mostly derived from the tame guinea-pig should exhibit the 
full power of its customary restrictive action because of a failure to set 
up wholly harmonious relations with these factors. This modifying 



SPECIES HYBRIDIZATION 227 

of the wild agouti pattern, therefore, lends additional support to the 
conclusion that these two agouti factors, although occupying homologous 
loci in the hereditary systems of the two species are different from each 
other. 

The Forms of Species Hybrids. — Thus far we have dealt with two 
species crosses which have given satisfactory indications of behavior 
essentially in accord with generally accepted Mendelian principles. 
The remainder of the chapter will be devoted to general considerations 
respecting species hybrids and to particular cases which do not give 
entirel}^ satisfactory evidence of Mendelian behavior. In common 
with most variety hybrids, species hybrids display marked uniformity 
in the first generation and equality of reciprocal crosses. Exceptions, 
however, occur to both these conditions and these we shall take up 
later in the discussion. 

With respect to the characters which they display species hybrids 
usually represent an intermediate condition as compared with the parents. 
We may refer this condition to a mixture of dominant factors derived 
from both parents and in* some cases to actual intermediate expression 
of contrasted allelomorphs, as is not uncommon in variety hybrids. 
The intermediacy of Fi in species crosses is a well-known phenomenon 
and is so common that it may be regarded as the rule. This condition 
was well known to the older hybridists, such as Kolreuter, Gartner, 
Naudin, and Focke, all of whom investigated extensive series of species 
hybrids with respect to the characters both of the immediate hybrid and 
of its progeny. The intermediate condition* however, is not universal, 
for examples are known of all conditions from that of strict intermediacy 
to a condition so nearly resembling one parent in certain cases that only 
slight character differences or sterility establish the existence of an 
actual cross. 

Intermediate species hybrids are so common that it seems super- 
fluous to call especial attention to them, nevertheless this will be done 
in order to point out the relation of the intermediate condition to other 
characteristic features of species hybridization. In the first place the 
intermediate condition is not associated with any particular degree of 
fertility in the hybrids. Partial sterility is a common characteristic 
of wide crosses, and in fact this sterility in some cases appears to be 
complete. The Antirrhinum species hybrids are intermediate in practi- 
cally all characters, but they are apparently completely fertile. Such 
cases are, however, uncommon in species hybridization, but neverthe- 
less a few others have been studied. Baur and Lotsy have reported 
other species hybrids in Antirrhinum which give fertile intermediate 
hybrids. 

Some species hybrids in Nicotiana are known to be very nearly com- 



228 GENETICS IN RELATION TO AGRICULTURE 

pletely fertile. East has reported investigations of a cross between 
A'', alata and N. langsdorffii. N. alata has flowers the corolla length 
of which averages about 82 mm., whereas the corolla length of flowers 
of A^. langsdorffii averages not over 22 mm,, so that A^. alata flowers are 
nearly four times as large as those of N. langsdorffii. In addition to 
these differences there are other distinct differences between the two 
species. Nevertheless examination indicated that there was little, if 
any, diminution in fertility in Fi. A few other species hybrids in this 
group of Nicotiana give highly fertile Fi hybrids, for example A'', alata X 
A^. sandercB and A'', langsdorffii X A'', sanderce. There appears to be 
little reason for not regarding these as species hybrids, although it should 
be stated that some investigators feel inclined to restrict the species 
concept to forms which display a certain degree of partial sterility in Fi. 
Such a line of separation must, however, be purely arbitrary since it 
can be shown that fertile species hybrids merely represent one of the 
extremes in a continuous series extending from complete fertility to 
complete sterility. 

Since partial sterility is such a characteristic feature of species hybridi- 
zation, it is not surprising to find that diminution in fertility is not 
associated with any particular kind of character expression in the hybrids. 
Intermediate hybrids as well as those which more or less resemble one 
of the parents usually, therefore, display a considerably diminished 
fertility. Not much has been done with such hybrids for aside from 
exceptional instances sterility presents at once a bar to their further 
analysis and to their use for economical purposes. A familiar example, 
the mule, a cross between Equus cdballus and E. asinus, has given no 
authentic case of the production of offspring, although produced for many 
centuries under domestication and in vast numbers. Among plants so 
many examples occur that it is of no advantage whatever to attempt 
an enumeration of them here. The student who is particularly inter- 
ested in such matters will find that excellent compilations of species 
hybrids in plants have been made by Gartner and Focke; and Ackermann, 
Przibram, and Rorig have performed a similar service for the animal 
kingdom. 

In tobacco a large number of species hybrids occur which give partially 
sterile intermediate hybrids. The genus Nicotiana had been much 
employed in hybridization investigations providing as it did the first 
instance of hybridization in the plant kingdom when in 1760 Kolreuter 
crossed A'', rustica and A'', paniculata. The hybrid thus obtained was 
intermediate in its characters and was only slightly fertile. Varying 
comments have been made as to the exact expression of the characters 
of this hybrid as compared with those of its parents, but apparently 
careful scrutiny reveals the influence of both parents in practically 



SPECIES HYBRIDIZATION 229 

every character, although to varying extents in different characters. 
All, however, have found it relatively infertile, although among some 
hundreds Lotsy, in one experiment, discovered one plant which possessed 
a rather unusual degree of fruitfulness. 

Although the condition of intermediate character expression includes 
by far the majority of species hybrids, there are some notable exceptions 
which very closely duplicate the set of characters of one parent almost to 
the exclusion of those of the other. This fact was recognized even by 
the older investigators, for Gartner states that any condition may be 
obtained from that of strict intermediacy to a condition so closely re- 
sembling one parent as to be distinguished from it only by increased 
vigor and partial steriUty. Gartner found examples of dominance of 
one parent particularly striking in some Nicotiana crosses. Thus N. 
paniculata X N. langsdorffii is reported to give a hybrid form almost 
indistinguishable from N. langsdorffii and N. suaveolens X N. macrophylla 
is predominantly N. 7nacrophylla in its character expression. Later in 
this chapter we shall describe crosses between N. sylvestris and a series 
of varieties of A^. tahacum which constantly yield hybrids resembhng the 
particular tahacum variety used in crossing. Curious instances of such 
predominance of one type are reported for triple hybrids. Thus N. 
rustica X N. paniculata polhnated with N. angustifola gives plants closely 
resembling A^. angustifola; if the same hybrid is pollinated with N. 
glutinosa it produces plants closely resembling N. glutinosa. 

There are authentic instances of species crosses which do not give 
equivalent results in reciprocal crosses. It is a common observation 
that some species crosses may be made in one way only. Crosses be- 
tween wheat and rye are sometimes successful when wheat is the female 
parent, but the reciprocal cross has never been obtained. But usually 
when a cross is possible in both directions the reciprocal hybrids are prac- 
tically indistinguishable. Among exceptions to this rule are crosses 
between Digitalis purpurea and D. lutea, strikingly different species, 
which constantly give hybrids resembling the female parent. In Oeno- 
thera such results are particularly common, and de Vries and others have 
investigated a number of such cases. A typical case is that of 0. biennis 
and 0. muricata which give strongly patroclinous hybrids in reciprocal 
crosses. The fact, however, that these hybrids breed true in further 
generations introduces a complication which places us on our guard 
against the operation of some as yet undiscovered factors. We can under- 
stand why reciprocal crosses should give different results, when there 
are differences in chromosome number or content in the two sexes as is 
generally the case among animals, but in plants it is more difficult to 
assign a reason for this type of behavior aside from a few cases in which 
apogamy is known to occur. It is, therefore, necessary for us to accept 



230 GENETICS IN RELATION TO AGRICULTURE 

these cases with some reservations, looking to the future for experimental 
investigations which will provide us with a satisfactory explanation for 
them. 

The Vigor of Species Hybrids. — The increased vigor displayed by 
species hybrids has been frequently commented upon by investigators 
from the time of Kolreuter down to the present. In 1849 Gartner in his 
general treatment of this subject in species crosses especially notes that 
the luxuriance of hybrids frequently expresses itself in an unusual develop- 
ment of practically all plant parts. He also cites a considerable num- 
ber of the earlier hybridists who have noted this increased vigor, among 
them Kolreuter, Sageret, Berthollet, Herbert, Mauz, and Lecoq. Hy- 
brids which up to that time had been particularly noted for this sort of 
vigor represented such a large number of different famihes that there could 
be no question as to the generality of the phenomenon. For increase in 
length of stem Gartner notes especially Verhascum lychnites X V. thapsus 
which grows to a height as great as 15 feet; Althcea cannabina X A. 
officinalis which sometimes attains a height of 12 feet; Malva mauritiana 
X M. sylvestris which attains a height of 11 feet; Digitalis purpurea X D. 
ochroleuca which grows to a height of 10 feet; and finally Peiwnza nyctagini- 
flora X P. phoenicea and Lobelia cardinalis X L. syphylitica which attain 
a height of 3 to 4 feet, a significant increase as compared with their 
low-growing parents. Often the vigor is expressed in a general increase 
in size throughout as appears to be particularly true of hybrids between 
different species in the genera Mirabilis and Datura. In Nicotiana a 
number of hybrids such as N. suaveolens X A^. macrophylla, N. rustica X 
N. 7narylandica, and many others display such general hybrid vigor 
sometimes to a very marked extent. Tropceolum majus X T. minus, a 
hybrid of the tall and dwarf nasturtiums of the garden is another notable 
instance of hybrid development. Gartner also records many interesting 
ways in which this hybrid vigor expresses itself. Thus certain hybrids 
in Dianthus, Lavatera, Lobelia, Lychnis, Geum, and Penstemon while 
not displaying notable increases in vegetative vigor lend themselves 
much more readily to vegetative propagation than do their parents. In 
some cases the hybrids show an unusual tendency to produce side 
branches and suckers, and in other cases still other outlets of this hybrid 
vigor are found. 

Not all species hybrids, however, display hybrid vigor, and many 
indeed show a strikingly weakened condition accompanied by much less- 
ened vegetative vigor. In tobacco several species hybrids show less- 
ened vegetative vigor, as for example, Nicotiana grandiflora X N. gluti- 
nosa, N. glutinosa X A'^. quadrivalvis, N. rustica X N. suaveolens, and A^. 
suaveolens X N. quadrivalvis. Similarly Verbascum blattaria X V. lych- 
nitis gives weakened hybrids. Consequently within the same genus 



SPECIES HYBRIDIZATION 231 

some species hybrids show marked increases in vegetative vigor, whereas 
others show just as marked decreases. 

Investigations since Gartner's time have simply extended observa- 
tions on the comparative vigor of parents and hybrids in species hybrids 
as well as in the less violent variety hybrids. Thus Focke who inves- 
tigated large numbers of species hybrids found many that were abnor- 
mally weak, but these usually represented rather wide crosses. Crosses 
between more closely related species, however, generally showed an in- 
creased vegetative vigor. The increased vegetative vigor, he regards as 
merely an extension of the same condition which Darwin had investigated 
in variety crosses, namely that crossbreeding is advantageous from the 
standpoint of the general growth of the forms involved. The idea that 
sterility may be the cause of this increased vigor is refuted on the one 
hand by the fact that some of the most vigorous species hybrids are also 
highly fertile, and on the other hand by the fact that most of the weak 
hybrid forms are nearly or quite sterile. 

East and Hayes have attempted to offer an explanation for these 
phenomena on the basis of heterozygosis. They have reached this con- 
clusion from extensive investigations of the effect of self-fertilization in 
maize and of cross-fertihzation in tobacco. In corn they have found, 
as we shall describe more in detail later, that continued self-fertilization 
results in the isolation of races which are very uniform as respects their 
character development, but which almost constantly show considerably 
decreased vigor of growth. This decrease in vigor is most rapid in the 
first generations and becomes less rapid as the races become more con- 
stant in their characters. Since the approach to constancy in char- 
acters may be regarded as evidence of approach to a homozygous condi- 
tion in this a normally highly heterozygous species, East and Hayes argue 
that the normal vigor of maize is largely an expression of its heterozygous 
condition and that the decrease in vigor is a consequence of reduction to 
a homozygous condition. This conclusion is in part confirmed by the 
evidence from crossing such homozygous strains of maize. The Fx 
of such crosses usually exhibits an immediate return to the vigor of the 
population from which the strains were isolated. However, it is not 
entirely clear why this behavior cannot be ascribed to the isolation of 
races possessing fewer dominant factors than most of the plants in the 
original population. When such races are crossed the original set of 
dominant factors would be reunited, and in consequence the normal 
vigor of the original population would be exhibited. 

Since the foregoing was written D. F. Jones has published an explana- 
tion of increased vegetative vigor of hybrids or ''heterosis," as it has 
been termed by Shull, which he has summarized as follows: 



232 GENETICS IN RELATION TO AGRICULTURE 

1. The phenomenon of increased growth derived from crossing both plants and 
animals has long been known but never accounted for in a comprehensible manner by 
anj^ hypothesis free from serious objections. 

2. The conception of dominance, as outlined by Keeble and Pellew in 1910 and 
illustrated bj^ them in height of peas, has had two objections which were: a. If hetero- 
sis were due to dominance of factors it was thought possible to recombine in genera- 
tions subsequent to the F2 all of the dominant characters in some individuals and all 
of the recessive characters in others in a homozj^gous condition. These individuals 
could not be changed by inbreeding, b. If dominance were concerned it was con- 
sidered that the ^2 population would show an asymmetrical distribution. 

3. All hypotheses attempting to account for heterosis have failed to take into 
consideration the fact of linkage. 

4. It is shown that, on account of linked factors, the complete dominant or com- 
plete recessive can never or rarely be obtained, and why the distributions in F2 are 
symmetrical. 

5. From the fact that partial dominance of qualitative characters is a universal 
phenomenon and that abnormalities are nearly always recessive to the normal condi- 
tions, it is possible to account for the increased growth in Fi because the greatest 
number of different factors are combined at that time. 

6. It is not necessary to assume perfect dominance. It is only necessary to accept 
the conclusion that many factors in the In condition have more than one-half the 
effect that they have in the 2ti condition. 

7. This view of dominance of linked factors as a means of accounting for heterosis 
makes it easier to understand: a, why heterozygosis should have a stimulating rather 
than a depressing or neutral effect; and b, why the effects of heterozygosis should 
operate throughout the Ufetime of the individual, even through many generations 
of asexual propagation. 

In order to extend their investigations to normally self-fertilized 
species, East and Hayes made numerous crosses between different 
species of Nicotiana. As a result they found, as indeed had previous 
investigators on this the favorite genus for hybridization studies, that 
the vigor of hybrids varied all the way from a condition so weak as to 
give embryos incapable of germination to a condition greatly exceeding 
in vigor that exhibited by either parent. Thus Nicotiana tabacum, 
the commonly cultivated tobacco, when crossed with A'^. sylvestris gives 
hybrids which exceed by 35 per cent, the average height of the parents 
and are estimated to be 20 per cent, more vigorous. Similarly N. 
tabacum, when crossed , with TV. rustica, gives hybrids exceeding by 80 
per cent, the average height of the parents, but when crossed with N. 
alata grandiflora, the hybrids are only about 10 per cent, of the average 
of the parents, both in height and vigor. About fifty species crosses 
within the genus were made by these investigators with results in re- 
spect to vigor which bore out those already known for various specific 
crosses. 

It is obvious from these results that stimulation in the hybrid is a 
result of certain specific interactions. East and Hayes regard those 
hybrids which show decreased vigor as evidence of such great differences 



SPECIES HYBRIDIZATION 233 

between parents that normal cell division is impossible. When on the 
other hand the differences are not great enough to obstruct normal cell 
division, the degree of stimulation is held to increase directly with the 
amount or kind of heterozygosis present. 

These conclusions, however, do not appear to be very firmly estab- 
lished on the experimental side for by no means the only explanation 
has been offered. When we consider the recent work with Drosophila 
it is clear that many factor differences are known which in addition to 
resulting in some definite character distinction display a rather ill-defined 
effect in decreasing vigor. Thus the factor for white eye color in addi- 
tion to determining white eyes has such an effect on the viability of 
the white-eyed phenotype that in segregation this class never comes 
up to Mendelian expectations. Similarly other factors have definitely a 
weakening effect in vigor, in sterility, and in other characteristics. This 
effect, also, is apparently cumulative, so that in Drosophila strains 
containing many recessive factors almost invariably must be carried 
on in a heterozygous condition on account of their low viability. Here 
very evidently the increased vigor of the heterozygous strains is to be 
attributed to a recombination of the dominant factors normal to the 
wild type, for the heterozygous forms display the characters of the 
normal wild type and a size and vigor approximating that of the homo- 
zygous wild type. Similarly in corn the occurrence of open pollination 
makes it possible for a relatively large number of such factors which 
lower vigor to exist in a variety. These show their effects in marked 
degree only on self-fertilization for such self-fertilization automatically 
results in a rather rapid reduction of strains to a homozygous condition. 
If the number of recessive factors affecting vigor is fairly large then it is 
evident that the mathematical probability of isolating some of them in 
continued self-fertilization is relatively great, but the chances that the 
same ones will be isolated in different pure strains is relatively slight. 
It follows that usually pure strains resulting from continued self-fer- 
tilization will display lessened vigor and productiveness, and that dif- 
ferent strains isolated in this fashion will give hybrids approximating 
the normal condition of fertility and productiveness. The increased or 
even decreased vigor of species hybrids of the wider type appears, there- 
fore, to belong to a distinctly different category for which we are not 
yet fully prepared to provide an explanation. To suggest that the 
increased stimulation depends on the specific interactions which occur 
between two different contrasted hereditary systems is, confessedly, 
falling back on a less definite explanation, but one which does not appear 
improbable when viewed in the light of our knowledge of the unexpected 
relations which certain factor combinations display when brought 
together. 



234 GENETICS IN RELATION TO AGRICULTURE 

Sterility in Species Hybrids. — A common phenomenon of species 
hybridization is the marked degree of sterility which is exhibited. This 
fact of steriUty in speices hybrids has led certain investigators, par- 
ticularly Jeffrey, to lay great stress upon partial sterility as an evidence 
of hybrid character. This contention may be valid for a majority of 
cases, but obviously it would not follow even if all species hybrids dis- 
played partial sterility, that all cases of sterility are to be referred to 
hybridity. Specifically many instances are known for which simpler 
and more satisfactory explanations suffice. Thus Bateson has recorded 
a case of contabescence in the anthers of sweet peas which is strictly 
due to the presence of a definite factor for contabescence. The ratios 
obtained are approximately 3 : 1 ratios, contabescence being recessive, 
and moreover, the factor for contabescence is definitely linked with 
other factors, so that in every respect the factor analysis of this case of 
sterility is wholly satisfactory. In Drosophila similarly some cases of 
sterility are definitely referable to the action of specific factors which 
sometimes have effects so marked that strains homozygous for the 
factors in question cannot be maintained. This condition is well illus- 
trated by flies which are homozygous for the factor for rudimentary 
wings. Such flies are practically never fertile. Many other instances 
are known where slight effects on fertility result from factors which are 
intimately concerned in the expression of other characters. A some- 
what different type of sterility, but one which is also definitely established, 
is that which Bridges has reported for the males of Drosophila ampelophila 
which lack the F-chromosome. The evidence upon which this case of 
sterility is based appears to be conclusive, and to demonstrate that while 
the male Drosophila lacking the F-chromosome may develop a normal 
soma, it cannot produce functional germ cells. The sterility of wide 
crosses, however, appears to belong to a distinct category, an explanation 
for which we shall endeavor to give later on in this chapter. At this point 
we shall only take up some of the types of sterility displayed in such crosses. 

At the outset it may be well to note that the degree of sterility dis- 
played by hybrids varies from complete fertility to complete sterility. 
It is, therefore, readily apparent that sterility in hybridization as a 
means of species differentiation gives no natural divisions, but that 
arbitrary ones must be erected depending upon the degree of sterility 
displayed. Moreover, other factors such as those noted above com- 
plicate matters and render it extremely difficult to decide where to draw 
the line. Here again, therefore, the search for a universal species in- 
dicator has met with failure. From a genetic standpoint this is as it 
should be for it merely indicates that races of plants and animals display 
all degrees of genetic differences from simple differences in isolated factors 
to complex differences in entire series of factors. 



SPECIES HYBRIDIZATION 



235 



Sterility in crosses between apparently good species may be at times 
almost completely lacking. Thus the crosses between Antirrhinum 
majus and A. molle and some other crosses made by Baur within the 
genus Antirrhinum proved fully fertile. The same condition has been 
found in other species crosses. Thus Nicotiana alata grandiflora and 
N. langsdorffii, although they differ strikingly in their characters, give 
hybrids which are about as fertile as the parents. Certain orchid 
crosses are also reputed to display a high degree of fertility, but on the 
whole crosses between good species very rarely show even an approxi- 
mation to the full degree of fertility, and this is true of both plant and 
animal hybrids. 

The sterility displayed by species hybrids may not always be equiva- 
lent in both sexes. Thus one of Baur's Antirrhinum crosses, that of A. 
majus X A. siculum proved completely sterile as far as the production 
of good ovules is concerned, but some good pollen grains are produced 
which can be used in back crosses to the parents. In the case of Cavia 
porcellus X C. rufescens, we have already noted that the males are 
sterile and the females fertile. Detlefsen attempted to follow out the 
inheritance of fertility in this case, and attacked the problem from 
many angles. The fertility of the females appears to be complete, 
since the Fi females produce litters of approximately the average number 
of young of those of the two parent species. The offspring of the 
hybrid females when crossed back to the tame guinea-pig again produce 
fertile females and sterile males. With each successive back cross to 
the tame guinea-pig the percentage of fertile males rises in a fairly 
regular fashion as is shown in Table XXXVII. Detlefsen points out for 
this case that the assumption that the wild species carries eight disturbing 
dominant factors gives a very close agreement with the observed re- 



Table XXXVII.— Percentages of Hybrid Offspring with Many Mobile Sperm 

IN Matings of Female Hybrids with Tame Guinea-pigs, and Female 

Hy'brids with Fertile Male Hybrids {After Detlefsen) 



Generation 


Offspring of female hybrids 
and guinea-pigs 


Offspring of female hybrids 
and fertile male hybrids 


Calculated for 


of females 


Number 


Percentage with 
many mobile sperm 


Number 


Percentage with 
many mobile sperm 


eight factors 


Fi 

F2 


1 

8 
49 
99 
150 
49 
15 


00.0 
00.0 
14.3 
33.3 
60.7 
69.4 
73.3 


1 

2 
7 

17 
11 

1 


00.0 
00.0 
14.3 
58.8 
63.6 

100.0 


00.0 
00.4 


Fs 

Fi 


10.0 
34.4 


F, 

Fe 


59.7 
77.6 


Fv 


88.2 











236 GENETICS IN RELATION TO AGRICULTURE 

suits. The Fi hybrids would then be of the genetic constitution AaBb- 
CcDdEeFfGgHh. Such individuals produce gametes of the constitu- 
tion ahcdefgh only once in 256 times, so that when crossed back to tame 
guinea-pigs which produce only gametes bearing the recessive factors, 
0.4 per cent, of the males should be fertile. The percentage of fertile 
males in successive generations of back crossing should then increase pro- 
gressively as shown in the last column of Table XXXVII. As Detlefsen 
himself, however, points out the close agreement of these calculated 
figures with those actually observed is misleading as an indication of the 
significance of the analysis, for it is doubtful whether simple segregation 
of Mendelian factors provides an explanation of the entire phenomena. 
It is rather strange in fact that only the males display this sterility, 
and it is of interest to note, as Detlefsen points out, that several other 
analogous instances of male sterility in animal species hybrids are known. 
The yak, Bibos grunniens, crossed with the domestic cow. Bos taiirus, 
gives fertile female and sterile male offspring. Similarly the gayal, 
Bihos frontalis, the gaur, Bibos gaurus, and the American bison, Bison 
americanus, have been crossed with domestic cattle and have given fertile 
female and sterile male hybrids. There is strong evidence that hybrids 
of the banteng, Bibos sondaicus, and the zebu. Bos indicus, display similar 
relations. When we consider the physiological relations between factors 
and particularly the significant fact that probably no crossing-over 
occurs in the males of this species, we feel inclined to attribute the male 
sterility to other causes than to a mere sorting of factors having to 
do with fertility. 

Partially Sterile Hybrids of Wheat and Rye. — Thus far cases have 
been considered in detail in which the species hybrids display a consid- 
erable degree of fertility. At the other extreme stands a series of hybrids 
which display sterility which is nearly but not quite complete. Such are 
the hybrids between wheat and rye which Jesenko has subjected to 
thorough experimental study. There can be no question that wheat and 
rye are distinct species, in fact they have been universally assigned to 
different genera. They seem to represent about the extreme limitations 
of effective hybridization. Jesenko and others have been able to obtain 
hybrids between wheat and rye only when wheat is used as the female 
parent, consequently we are unable to compare the results of reciprocal 
hybridization in this case. Even pollination of wheat with rye is suc- 
cessful only about six times in one thousand as Jesenko found in over six 
thousand trials with different species and varieties. The Fi hybrids were 
intermediate in general characters, although the relations of dominance 
displayed in variety crosses was preserved in the species crosses. In 
Fig. 100 is illustrated one of these hybrids and its two parents. The in- 
creased size of the spike as compared with those of either parent is par- 



SPECIES HYBRIDIZATION 



237 



ticularly striking. These Fi hybrids are completely sterile with their own 
pollen. However, it was possible by pollinating the hybrids either with 
pollen from wheat or rye to obtain a few viable seeds. For wheat pollina- 
tion the ratio of success was about 3 in one thousand; for rye only one 
plant was secured from nearly five thousand trials. The pollen grains of 
the hybrids were apparently completely non-functional, and cytological 
examination indicated prevailingly irregular divisions and behavior in 
their production. 

The product of back-crossing the Fi hybrids to wheat gave plants 





•1 
J, 








I 


^U 


1 


'1 1 


J 


1 


' 


'i^ 






i 


1 


1 

i 
1 

j • / 




iV 


\ ' 


A 


B 


C 




Fig. 100.— Sterile hybrids be- 
tween wheat and rye, A, the wheat 
parent; C, the rye parent, and B, 
the Fi hybrid between them. 
(After Jesenko.) 



Fig. 101. — Sesqui-hybrids from thei^i wheat X 
rye crossed back to wheat. (After Jesenko.) 



very similar to wheat. This is illustrated in Fig. 101. Although all 
these plants resembled wheat in their general characters, they neverthe- 
less showed wide differences from one another, not only in morphological 
characters but in physiological ones such as fertility as well. A few of 
the plants were totally sterile, but some of them were more or less fertile 
and in general those were most fertile which most closely resembled the 
wheat parent. In the next following generation, the progeny of those 
plants which were most fertile consisted of plants which were apparently 
pure wheat and completely fertile and plants which were less like wheat 
and showed lessened fertility as the resemblance to wheat decreased. 



238 GENETICS IN RELATION TO AGRICULTURE . 

For a few particular characters, Jesenko was able to establish close ap- 
proximation to a Mendelian analysis, so that it can scarcely be doubted 
that in the sorting out of the factors to establish the constant races of 
further generations, the phenomena displayed were such as to indicate 
clearly the operation of a Mendelian mechanism. 

But when we consider the phenomena in the light of the characters 
involved, then it may be seen that the results obtained are truly remark- 
able. Wheat and rye differ strikingly in their characters and the recovery 
of approximately the parental form so often in these back-crosses is out 
of the question from a strict Mendelian viewpoint, if all combinations 
are assumed to survive. 

As an explanation of these phenomena, Jesenko calls attention to the 
fact that there are eight chromosomes in the germ cells of rye and wheat, 
so that in the formation of gametes in the i^i some will possess eight wheat 
chromosomes, others seven wheat and one rye, and so on. When back- 
crossed to wheat, therefore, union with those gametes which contain only 
wheat chromosomes or at most two or three rye chromosomes results in 
wheat-like plants which are fertile, whereas greater proportions of rye 
chromosomes results in plants which are less like wheat and sterile. 
Similarly, as Jesenko in fact found, pollination with rye results in plants 
resembling rye, because of the union of the rye pollen with gametes 
which contain all or nearly all rye chromosomes. The sterility in these 
hybrids, therefore, Jesenko regards as the consequence of the inharmoni- 
ous action of a "plasma" built up of large proportions of both rye and 
wheat elements. 

Partially Sterile Hybrids in Nicotiana. — A similar state of affairs 
has been found to exist in hybrids between various varieties of Nicotiana 
tahacum, the commercial tobacco, and N. sylvestris, a very different 
species. N. tahacum occurs in a very large number of distinct varieties 
some of which are so different that they could justly lay claim to recogni- 
tion as distinct species. Goodspeed and Clausen have studied the 
hybrids of a number of N. tahacum varieties with N. sylvestris and have 
found that in all cases the Fi hybrid duplicates very closely the total set 
of characters of the particular tahacum variety used in the hybrid save 
on a very much enlarged scale, for these hybrids are conspicuous for the 
increased vigor due to hybrid stimulation. In Fig. 102 is illustrated a 
typical plant of N. sylvestris. N. sylvestris is a monotypic species and 
has been grown under cultivation for over thirty years without producing 
any distinct varieties. It is a strikingly beautiful plant with its stout, 
erect growth; stiff, broad ascending leaves; and its star cluster of long pure 
white flowers. Nothing even approximating its flower characters 
occurs in the numerous varieties of N. tahacum,, in fact it belongs to a 
totally distinct section of the genus Nicotiana. In spite of its distinct 



SPECIES HYBRIDIZATION 



239 



characters, however, it crosses freely with members of the tabacum 
group, and yields reciprocal hybrids which are equivalent throughout. 
In Fig. 103 on the right is illustrated a plant of N. tabacum angustifolia 
and beside it the Fi hybrid with A^. sylvestris. The figure shows clearly 
how faithfully the characters of the A'', tabacum parent are reproduced in 
the hybrid. The leaves are long, narrow and petioled, the upper ones 
strap-like and pendant, the flowers are narrow and have narrow, sharply 




Fig. 102. — Typical plant, of Nicotiana sylvestris. 



pointed lobes — these and the general habit of growth are all characters 
clearly referable to the A^. tabacum parent. A very different variety of 
tabacum, such as the variety known in the University of California Bot- 
anical Garden cultures as A^. tabacum "Cuba" gives corresponding results. 
This variety is tall and bears white flowers many of which are quadrimer- 
ous instead of pentamerous as is normally the case in Nicotiana, These 
characters are faithfully reproduced in the hybrid with sijlvestris as is 
shown in Fig. 104. A^. tabacum ''Cuba" is peculiar among the tabacum 
varieties in its ability to develop seed capsules in the absence of fertiliza- 
tion, and these may sometimes contain a few viable seeds. This is appa- 



240 



GENETICS IN RELATION TO AGRICULTURE 



rently a recessive character in crosses with A^. tabacum varieties which 
display a normal behavior in this respect, but it is manifested in the i^i 
hybrids with N. sylvestris in the remarkable way in which this hybrid 
retains its seed capsules, although there are very few or no seeds in them. 
Since all the other Fi hybrids of tabacum varieties and sylvestris shed their 
flowers, often before the corolla has withered, this feature has very 
conspicuously characterized the Fi hybrids of A^". tabacum "Cuba" and 
sylvestris. 





* f ' ^l-') 










Fig. 103. — Nicotiatia sylvestris (left), A'', tabacum angustifolia (right) and the Fi hybrid 
(center). (After Goodspeed and Clausen.) 

The significant feature of these hybrids, however, is the hereditary 
behavior which they display. They are almost completely sterile, but 
if the plants are grown under reduced conditions of culture and the flowers 
are hand pollinated with pollen from either of the parent species, a few 
seeds are set, but not more than about 1 per cent, of the number ordi- 
narily produced by the parents. If N. sylvestris pollen is used to polli- 
nate the Fi, the sesqui-hybrids thus obtained are of diverse types, most 
of them abnormal, but about 10 per cent, closely approximate N. 



SPECIES HYBRIDIZATION 



241 



sylvestris in all their characters. These latter plants are fertile and in 
succeeding generations give offspring which to all indications are pure 
sylvestris individuals. Similarly when pollen of the N. tabacum parent 
is used, the sesqui-hybrids are of a variety of forms, but all approximate 




Fig 104. — Nicoliatia tabacum "Cuba" (left) and its Fi hybrid with A'', syhestris (right). 
(After Goodspeed and Clausen.) 

the N. tabacum parent in their characters and no one could determine by 
studying them that they were only once removed from A'', sylvestris. Those 
which most closely approximated the N. tabacum parent in morphological 
characters are also most fertile and give rise to fertile races which do not 
differ significantly from the A^. tabacum parent. 

16 



242 GENETICS IN RELATION TO AGRICULTURE 

The behavior is truly remarkable when viewed in the light of modern 
Mendelian conceptions. The number of character differences between 
the two forms is very considerable, and the recovery of the parental 
forms with almost unimpared fertility is so frequent that subsidiary 
assumptions must be made to account for them on a Mendelian basis. 
Goodspeed and Clausen, therefore, have developed the conception of 
Mendelian reaction systems for an explanation of these phenomena. 
According to this conception the normal functioning of a gametic or zy- 
gotic set of factors depends upon the harmonious interrelations which 
the factors maintain with one another. The uniform resemblance of the 
Fi hybrids of A^. tabacum varieties with N. sylvestris to the N. tahacum 
varieties is held in these cases to indicate that the N. tabacum set of 
factors is dominant as a Mendelian reaction system to the set of factors 
contributed by N. sylvestris. The fact that these hybrids so completely 
resemble the N. tahacum parent indicates that the elements of the N. 
sylvestris system are throughout unable to interact normally with those 
in the opposed N. tahacum system. It is for this reason that a reces- 
sive factor which is practically completely swamped in Fi intervariety 
crosses in N. tabacum, expresses itself so strongly in the Fi hybrids with 
N. sylvestris for, if the corresponding element of the N. sylvestris system 
were unable to interact with the elements of the dominant reaction 
system, then it is clear that although the factor is dominant, the corre- 
sponding character cannot possibly express itself in the individual. 

The haploid number of chromosomes in these Nicotiana species and 
varieties is probably twenty-four. Consequently the recombination 
series is given by the expansion of the expression (1 + l)^*. Only one 
gamete in 16,777,316 would carry only N. tahacum chromosomes and 
the same proportion would hold for gametes carrying only N. sylvestris 
chromosomes. This is on the assumption that no crossing-over occurs 
in the formation of gametes in the Fi hybrid. If crossing-over should 
occur normally the proportion of pure N. sylvestris or N. tabacum gametes 
would then be correspondingly reduced. The further assumption is 
also tacitly made that there are some factor differences between N. 
tabacum and N. sylvestris in every chromosome, which is in all probability 
correct when we consider the striking differences between the two species. 
Accordingly the results of the back-cross with N. sylvestris which gives 
a relatively high percentage of what are apparently pure N. sylvestris 
plants are exceedingly significant. Developing the reaction system 
hypothesis, it would appear that, if the N. tabacum and N. sylvestris 
systems display a high degree of mutual incompatibility, any gamete 
containing elements derived from both systems would give a reaction 
system subject to profound disturbances incident upon the inharmonious 
relations set up between the N. tabacum and N. sylvestris elements. If 



SPECIES HYBRIDIZATION 



243 



the admixture be relatively slight, the inharmonious elements may not 
greatly affect the workings of the reaction system, and there would result 
individuals showing practically the entire set of characters of one or the 
other parent, and such individuals would be fully fertile. A slightly 
greater proportion of inharmonious elements in the reaction system would 
result in such profound disturbances in its functioning as to produce the 
abnormal individuals of various kinds which make up so large a propor- 
tion of the progeny from such parentage. When the proportions of 
inharmonious elements in the gametes becomes still greater, they fail to 
function at all. It is upon the formation of such non-functional gametes 
or the attempt to produce them, that the partial sterility of the hybrid 
depends, and since in this particular case these form by far the greater 
proportion of gametes, the hybrid is very nearly completely sterile. 

The relations may be illustrated by Table XXXVIII which represents 



Table XXXVIII. 



-Recombination Series in Gametes of Fi of A^. tahacum X A^. 
sylvestris 



Condition of 
gametes 


Tabacum: 

sylvestris 

chromosomes 


Proportionate 

number of 

gametes 


Piogeny when 

pollinated with 

A'^. tahacum 


Progeny when 

pollinated with 

A', sylvestris 




24:0 


1 


Plants resembling 


Plants resembling 




23:1 


24 


the N. tahacum 


the Fi and ab- 


Functional 


22:2 


276 


parent and of vari- 


normal plants but 




21:3 


2,024 


ous degrees of 


all nearly com- 




20:4 


10,626 


fertility 


pletely sterile 




19:5 


42,504 








18:6 


134,596 , 








17:7 


346,504 








16:8 


2.36,321 








15:9 


1,307,-504 








14:10 


1,961,256 








13:11 


2,496,144 






Non-functional . 


12:12 
11:13 
10:14 
9:15 
8:16 
7:17 
6:18 


2,705,456 

2,496,144 

1,961,256 

1,307,504 

736,321 

346,504 

134,596 


No viable seeds 


No viable seeds 




5:19 


42,504 


Plants resembling 


Abnormal, infertile 




4:20 


10,626 


the Fi hybrid and 


plants and fertile 




3:21 


2,024 


nearly completely 


plants closely re- 


Functional 


2:22 


276 


sterile 


sembling N. syl- 




1:23 


24 




vestris 




0:24 


1 







244 GENETICS IN RELATION TO AGRICULTURE 

the recombination series obtained in the Fi hybrid on the assumption that 
the chromosome mechanism is operating normally and there is no crossing- 
over. Neither of these assumptions is correct, but the table will show 
the principles involved in the production of the progeny by back-crossing. 
If it be assumed that the presence of not more than five N . sylvestris 
chromosomes in a system containing mostly N. tabacum chromosomes 
or correspondingly not more than five A^. tabacum chromosomes in a 
system containing mostly N. sylvestris chromosomes will not com- 
pletely disturb the relations within the systems to the point of failure to 
function at all, then about 0.7 per cent, of the gametes will be functional 
and 99.3 per cent, non-functional. This accounts for the high degree 
of sterility displayed by Fi. Pollinated by A'', tabacum those gametes at 
the A^. tabacum end of the series produce some plants which closely resem- 
ble the N. tabacum parent and are fertile, and others less fertile and resem- 
bling the A^. tabacum parent somewhat less. Conceivably some of these 
give abnormal forms such as have been observed in the cultures. The 
A'', sylvestris end of the recombination series pollinated with N. tabacum 
gives sterile hybrids approximating the Fi in their characters and some 
of these might likewise be abnormal. On the other hand when the A''. 
tabacum end of the series is fertilized by A^. sylvestris, sterile individuals 
result which resemble the Fi and perhaps where there is any missing link 
in the chain of tabacum chromosomes, the resulting individuals are ab- 
normal. The A^. sylvestris end of the series, however, gives fertile in- 
dividuals closely resembling A'^. sylvestris and perhaps abnormal indi- 
viduals which have a tendency to resemble A^. sylvestris. The high pro- 
portion of fertile individuals resembling the parents in either case depends 
on the selective elimination of the greater proportion of the gametes 
which contain elements derived from both parents. The conception 
then that recombination gametes must form harmonious reaction sys- 
tems in order to function accounts in these nearly sterile hybrids for 
the high degree of sterility, for the quick recovery of either parent by 
back-crossing, and for the recovery of full fertility in subsequent genera- 
tions upon return to the parental type. It is a curious consequence 
of these phenomena that it is easier to recover the exact parental types 
from hybrids of A", sylvestris and A'', tabacum than from intervarietal 
hybrids of A'', tabacum, which are fully fertile and display all manner of 
recombinations. 

Species Hybridization in (Enothera. — Curious results have been 
obtained in (Enothera in which genus considerable attention has been 
given to the results of hybridization of a large number of different species. 
Since these results have often been cited as evidence of non-Mendelian 
behavior, it is well to consider some of them in detail. De Vries par- 
ticularly has made a thorough study of almost every conceivable com- 



SPECIES HYBRIDIZATION 245 

bination of species within the genus, and also of the mutants of 0. 
lamarckiana with the parent species, with one another, and with other 
species. 

As an example of the type of behavior displayed by these hybrids we 
shall take the results of intercrossing lamarckiana and its two mutant 
derivatives ruhrinervis and nanella. When lamarckiana is crossed with 
rubrinervis, the phenomena are as outlined below: 

lamarckiana X ruhrinervis 



lamarckiana subrobusta 



lamarckiana subrobusta rubrinervis 

The Fi consists of two forms in about equal proportions, lamarckiana 
and subrobusta, the latter a form intermediate between lamarckiana and 
rubrinervis. In subsequent generations, the lamarckiana individuals 
breed true, but the subrobusta individuals produce both subrobusta and 
rubrinervis, the latter breeding true. To these results we may add those 
obtained by crossing lamarckiana and nanella, the dwarf mutant of 
latnarckiana. This cross gives in Fi approximately equal numbers of 
lainarckiana and naviella and both forms breed true in subsequent 
generations. Finally to complete the triangle we may consider the 
results of hybridization of rubrinervis and nanella which are given below 
in the form of a diagram. 

nanella X rubrinervis 



lamarckiana subrobusta 



lamarckiana rubrinervis subrobusta dwarfs 

The percentage of subrobusta individuals in the Fi of this cross is usually 
considerably below 50 per cent. In subsequent generations the subro- 
busta individuals segregate in the same fashion as those of the Fi. The 
dwarfs obtained in this experiment unite the characters of r-ubrinervis 
and nanella and are consequently designated rubrinervis nanella to dis- 
tinguish them form the true nanella. Like the lamarckiana and rubri- 
nervis individuals, they breed true in subsequent generations. The actual 
results of this series of experiments are given in Table XXXIX, from 
which data on those forms which bred true is omitted. It is at once ap- 



246 



GENETICS IN RELATION TO AGRICULTURE 



Table XXXIX. — Results of Various Matings of rubrineruis (R.) and nanella (N.) 

AND THE Forms Produced prom Such Matings (compiled from 

de Vries, '^Gruppenweise Artbildung") 



Parentage 



Number 
of plants 



Lamarckiana 



Rubrinervis 



Subrobusta 



»r , Rubri- 

^f"" nervis 
*"" nanella 



Nanella X rubrinervis 

Nanella X rubrinervis 

Rubrinervis X nanella 

(N. X R.) subrobusta 

(R. X N.) subrobusta 

{R. X N.) subrobusta 

(A'^. X R.) subrobusta 

{R. X N.) subrobusta 

Lamarckiana X R. nanella 

R. nanella X lamarckiana 

(A''. X R.) lamarckiana X nanella 
{N. X R.) lamarckiana X nanella 
(R. X N.) lamarckiana X nanella 
Nanella X (R. X A'^.) lamarckiana 
Nanella X (N. X R.) lamarckiana 
{R. X A^.) lamarckiana X R. 
nanella 

Nanella X (A'". X R.) subrobusta. . 
R. nanella X {R. X A''.) subrobusta 
(N. X R.) subrobusta X nanella... 
(N. X R.) subrobusta X R. nanella 
(N. X R.) subrobusta X R. nanella 
{R. X A''.) subrobusta X R. nanella 



105 

79 

70 

160 

160 

56 

230 

234 

152 

152 

266 

70 

112 

68 

27 

84 

45 
204 
138 
246 
214 
289 



Per cent. 

73 
59 
59 



3 

25 
86 
80 
76 
62 
55 



33 
51 



Per cent. 



10 
3 
34 
21 
15 



87 

16 
33 
20 
75 

72 
72 



Per cent. 

27 
41 
41 
80 
85 
52 
70 
73 
77 
32 



Per cent. 



10 

12 

14 

9 

12 



20 
43 
14 
20 
24 
38 
45 



51 
67 
29 
25 

28 
28 



parent that the phenomena exhibited, although complex, are very orderly; 
but no very consistent Mendelian interpretation has been advanced to 
account for all of them. 

The hypothesis of de Vries while ingenious does violence to many of 
our most cherished conceptions of the general nature of hereditary 
phenomena. De Vries assumes that pangens exist in three forms; 
active, labile, and inactive. Two pangens are concerned in the above 
series of forms, the rubrinervis pangen for strengthening of the vascular 
bundles and the nanella pangen for stature. These pangens exist in 
lamarckiana in the labile condition in which they occasionally change 
to the inactive condition and thus produce the corresponding muta- 
tions rubrinervis and nanella. Labile pangen X inactive pangen then 
gives according to de Vries in Fi the ascendency of either one or the 
other condition to the complete exclusion of the other form in later 
generations. Accordingly lamarckiana X nanella gives in i^i lamarck- 
iana and nanella which breed true in further generations. Similarly 



SPECIES HYBRIDIZATION 



247 



when lamarckiana is crossed with ruhrinervis, the rubrinervis pangen in 
lamarckiana is in the labile condition, but in ruhrinervis it is in the in- 
active condition. Here, however, a difficulty is introduced by the fact 
that the form corresponding to rubrinervis in Fy is intermediate between 
rubrinervis and lamarckiana, it is the form which de Vries calls sub- 
robusfa. Must we assume a fourth condition for the pangens in this 
form? An additional difficulty is introduced when we consider crosses 
of rubrinervis and nanella. Rubrinervis has arisen from lamarckiana 
by mutation, by a change of the labile rubrinervis pangen in lamarckiana 
into the inactive condition. But when rubrinervis is crossed with nanella, 
Fx consists entirely of lamarckiana and subrobusta plants. As we pointed 
out, crosses of nanella with lamarckiana show that the nanella pangen 



NN 



rubrinervis 




N'N' Nn 

lamarckiana subrobusta 

I 

N'N' NN Nn nn 

lamarckiana rubrinervis subrobusta nanella 

Fig. 105. — Results of crossing two "mutants" of CEnothera lamarckiana. 

in lamarckiana is in the labile condition. How, then, should this 
pangen have become inactive in rubrinervis which was supposedly de- 
rived from lamarckiana by a change in the rubrinervis pangen? For 
according to de Vries the behavior of the nanella pangen in such an 
experiment is illustrated in Fig. 105 in which the active pangen 
is designated by N, the labile pangen by A'"', and the inactive pangen 
by n. 

Those who have attempted to apply a rigid Mendelian analysis to the 
ffinothera phenomena have failed to do so without making assumptions 
which thus far remain beyond the limits of experimental verification. 
Nevertheless the work of such investigators as Heribert-Nilsson, Renner, 
Davis, and others demonstrates that Mendelian analyses may be applied 
to particular cases and that when the difficulties which occur in GEnothera 
are considered, the facts thus far discovered do not preclude an ex- 
planation on an essentially Mendelian basis. Davis in particular has 
pointed out that thus far no species of ffinothera has been found which 
will stand trial as of strict genetic purity. In all species apparently 
50 per cent, or more of the pollen grains are abortive and similar 



248 GENETICS IN RELATION TO AGRICULTURE 

proportions of the ovules are non-functional. To this category of facts 
must be added the high percentage of seed sterility which is common 
in the genus. If any of this pollen, ovule, and seed sterihty is selective, 
then obviously it will be impossible to analyze the progeny successfully, 
unless the exact nature of the non-functional gametes and zygotes may 
be determined. The importance of this point has been indicated in 
the explanation of the frequent occurrence of parental forms among 
the sesqui-hybrids of rye and wheat and of Nicotiana tabacum with A^. 
sylvestris, and it has been definitely established for many cases of albinism 
in plants and for peculiar sex ratios and consequent disturbances of 
Mendehan ratios in Drosophila. Until, therefore, a satisfactory account 
can be given of the difficulties which have been enumerated above it 
will be impossible on the one hand to offer a satisfactory Mendelian 
interpretation of the ffinothera investigations and illogical on the other 
hand to advance the results of these investigations as evidence of non- 
Mendelian inheritance. 

Moreover, considerable success has attended the efforts to produce 
by species hybridization strains of ffinothera which behave like lamarck- 
iana. It is not without significance that Davis has been able to pro- 
duce forms by crossing 0. biennis and 0. franciscana so much like 
lamarckiana as to be indistinguishable from it taxonomically. Tower 
also has taken pure species of Leptinotarsa, the Colorado potato beetle, 
and by mating them has produced strains which breed approximately 
true, but which under the stress of unusual conditions may throw off 
small percentages of aberrant forms. In his species crosses in Anti- 
rrhinum, Lotsy has reported the occurrence of races which give small 
proportions of aberrant forms. Since at present we have no certain 
knowledge that lamarckiana is not a form of hybrid origin and that its 
so-called mutants are not really segregants from a race possessing a 
peculiar hybrid constitution, these analogous cases assume considerable 
importance as an indication of the hne of attack which may be followed 
for an explanation of the Oenothera phenomena. 

Conclusions.— If we attempt to outline the present status of our 
knowledge of the phenomena of species hybridization, we see thus far 
no clear evidence of non-conformance to an explanation which is essen- 
tially Mendehan. The strict Mendelian explanation must be modified 
to take into account the peculiar relations which obtain in species hy- 
bridization. For an explanation of such relations the reaction system 
conception has been advanced. According to this conception the total 
set of factors in any species forms a reaction system in which the factors 
display harmonious interrelations with one another. Variety hybridi- 
zation, since it is concerned only with isolated differences in systems 
which are fundamentally identical, usually produces no disturbances in 



SPECIES HYBRIDIZATION 249 

the reaction system relations. Consequently strict Mendelian analyses 
may be applied to such phenomena, and the reaction system relations 
need not be considered. But when species are crossed we must look to 
reaction system relations to account for the fact that not every set of 
factors which can be obtained by recombination is capable of establish- 
ing the harmonious interrelations which are necessary for normal func- 
tioning in a reaction system. As a consequence species hybrids exhibit 
a peculiar set of phenomena including sterility, whether partial or com- 
plete, production of abnormal forms, and apparent lack of conformance 
to established principles of hybridization. Underlying all these surface 
phenomena, however, is a behavior essentially Mendelian, if we take 
Mendelism to include all those phenomena consequent upon the shuffling 
and recombination of factors which possess at least a relatively high 
degree of stability. Since any irregularities in the distribution of factors 
or chromosomes, which may be occasioned by the inharmonious re- 
lations within the hybrid reaction systems acting upon the chromosome 
mechanism, can hardly be considered to give rise to results which should 
not be included under the term Mendelism, it is very evident that simple 
assumptions such as we have outlined above will account for a con- 
siderable array of phenomena. 



CHAPTER XIII 



PURE LINES 



For half a century succeeding Darwin, it was assumed that by 
selecting a certain type of individual for propagation, the species or 
variety would be continually transformed in the direction of the selec- 
tion. Such a conception was a natural result of the widespread 
acceptance of Darwin's theory of the method of evolution and later 
of Galton's ''law of inheritance" as applied to selection. Experience 
seemed to bear out this idea also, inasmuch as continual selection of the 
best plants for seed and the best animals for mating was found to be 
profitable. But it was not until Johannsen decided to test the power of 
selection by keeping the pedigrees of individual plants and their descend- 
ants that the truth concerning the composition of varieties of cultivated 
plants became known. Heterogeneity within single botanical species 
had already been discovered, but that horticultural varieties were also 
heterogeneous but with respect to less easily distinguishable characters 
had not been realized. Definite knowledge concerning the composition 
of horticultural varieties threw light on the problem of selection by ex- 
plaining why continuous selection within a variety is necessary in some 
crops while it has little or no effect in the case of certain other crops. 
This discovery was of tremendous significance to genetics, particularly 
to breeding. For this reason the following account of Johannsen's 
classical experiments is based directly upon his own presentation of the 
matter. 

Discovery of Pure Lines. — Johannsen chose a certain brown variety 
of the common garden bean {Phaseolus vulgaris nana) known as the 
Princess bean. In 1901 he harvested 287 plants which had grown from 
selected seeds of very different sizes and of known weights. The har- 
vested beans from each plant were weighed separately. They were then 
divided into classes with an interval of 10 eg., the class center values 
ranging from 30 to 80 eg. Next he determined the mean weights of all 
the beans from the plants grown from mother beans falling in the first 
class (25-35 eg.) and similarly for the progeny of each of the groups 
of the mother beans. The result is shown in the following table. 



Weight of mother beans 


30 


40 


50 


60 


70 


80 






Mean weight of progeny 


37.1 


38.8 


40,0 


43.4 


44.6 


45.7 







250 



PURE LINES 



251 



These two series may be expressed in terms of percentage by multiplying 
each scries by a factor that will change the value of the middle class to 
100. The mean weight of all the mother beans was very nearly 50 eg. 
while that of the progeny is approximately 40 eg. Thus the first series 
is multiplied by 2 and the second by 2.5 giving the following result. 



Weight of mother beans 


6C 


1 Sf 


) 


100 


120 




140 


160 




1 ""' 








Mean weight of progeny 


93 97 


100 


108 


111 


114 


Now the deviation of each progeny class can be compared directly with 
the deviation of the mother class. 


Deviation of mother beans 


-40 


-20 


! 20 


40 60 






Deviation of mean weights of progeny . . — 7 


-3 





8 


11 


14 



Thus the ratios of the minus deviations of the progeny classes to the 
minus deviations of the mother classes are %o ^^^ %q, the mean of which 
is i%o or 0.163. Similarly for the plus deviations, %o, ^}io> ^%0 X H, 
0.303. The average of these two values is 0.233 which is about 3^ as 
compared with Gallon's observation of ^3 inheritance in size of seed in 
the sweet pea and stature in man. 

During these preliminary experiments, however, Johannsen noticed 
that plants grown from similar sized beans produced beans of very differ- 
ent sizes. Thus, for example, the plants grown from the largest mother 
beans (about 80 eg. in weight) yielded seeds of strikingly different sizes. 
The average weight of the seeds of these individual plants varied between 
35 and 60 eg. and when the weights of all the individual beans of this 
series were arranged in a frequency distribution it produced a series that 
differed considerably from the normal frequency distribution. The 
distribution of 598 seeds, all progeny of beans about 80 eg. in weight, 
when arranged in classes of 5-cg. intervals, was as follows : 

Classes 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 

Number of seeds 5 18 46 144 127 70 70 63 28 15 8 4 

Theoretical numbers 1 3 11 26 53 85 109 112 91 59 30 13 4 1 

M = 45.44 ± 0.43 eg.; a = 10.40 eg. 

Clearly this distribution if plotted would produce a skew polygon with the 
mode to the left of the theoretical mode. This observation caused 
Johannsen to have serious doubt regarding the biological justification of 
Gallon's law. For such a distribution did not appear to be the expression 
of only one "type"; on the contrary, it seemed more likely that the 
material was mixed. 



252 



GENETICS IN RELATION TO AGRICULTURE 



This state of affairs was the starting point of further critical study. 
In order to take account of the effect of selection supposedly in the 
opposite direction, he next examined the progeny of the smallest mother 
beans (about 30 eg.) and found that they displayed no such striking 
irregularity as did the progeny of the largest beans. (Possibly this was 
due to the fact that about 20 plants were grown from the smallest beans 
while the progeny of the largest beans came from only 11 plants.) The 
progeny seeds from the smallest mother beans were weighed individually 
and the data put in the form of a frequency table as in the former case. 

Classes 15 20 25 30 35 40 45 50 55 60 65 

Number of seeds 8 18 71 156 172 127 35 15 3 6 

Theoretical numbers 1 6 27 77 139 162 121 57 17 3 1 

M = 36.68 ± 0.30 eg.; a = 7.33 eg. 

This distribution does not indicate a mixture. Instead it suggests that a 
single original "weight type" of bean was set apart by selection in the 
minus direction. The general result of this preliminary study was cer- 
tainly a sort of confirmation of Galtonian regression ; but at the same time 
the doubt was aroused whether the original population was not a hetero- 
geneous mixture from which selection simply sorted out already existing 
"types". Hence came the question: Will selection of plus or minus 
variants within pure lines bring about the isolation of types and cause 
Galtonian regression? 

This question was answered the following year (1902). A series of 
19 pure lines was used for this investigation. Each of these pure lines 
originated from a single bean from the crop of 1900. In the fall of 1901 
each line was represented by the seeds of one plant. In 1902 he planted 
524 seeds. Every seed was given a number and each plant was harvested 
separately. Each pure line, each plant and every single bean was sepa- 
rately numbered. Thus each individual could be compared with every 
other individual. Johannsen first compared his material as a whole with 
the results of his preliminary study. Having recorded the weight of 
each bean, he arranged the data in groups corresponding to the classes 
of mother beans as in the previous year. 



Weight of mother beans 


20 


30 


40 


50 


60 


70 






Mean weight of progeny 


44.0 


44.3 


46.1 


49.0 


51.9 


56.1 


Number of progeny seeds 


ISO 


835 


2,238 


1,138 


609 


494 



Again he found about 34 inheritance and ^ regression of progeny on 
mother beans. He next divided each of these six groups of progeny 
beans into classes according to weight as shown in Table XL. 



PURE LINES 



253 



Table XL. — Showing Variation within Classes in a Population Composed 
OF Pure Lines. (From Johannsen) 





Classes of progeny seeds in eg. 


n 


M 




beans, eg. 


5 15 


25 


35 


45 


55 


65 


75 


85 


95 




15-25 




1 


15 


90 


63 


11 










180 


43.78 + 0.56 


7.47 


25-35 




15 


95 


322 


310 


91 


2 








835 


44.47 + 0.31 


9.03 


35-45 


5 


17 


175 


776 


956 


282 


24 


3 






2,238 


46.17 + 0.19 


8.93 


45-55 




4 


57 


305 


521 


196 


51 


4 






1,138 


48.94 + 0.28 


9.34 


55-65 




1 


23 


130 


230 


168 


46 


11 






609 


51.87 + 0.42 


10.42 


65-75 


5 


38 


5 


53 


175 


180 


64 


15 


2 

2 


— 


494 


56.03 + 0.45 


10.02 


Total 15-75 eg.. 


370 1,676 


2,255 


928 


187 


33 


5,494 


47.92±0.13 
• 


9.87 



It is true that each of the six progeny series corresponds closely to 
the normal frequency distribution. There is no distortion such as would 
be expected from mixed material. Nevertheless it becomes evident that 
the material is heterogeneous as soon as the data are arranged by pure 
lines as shown in Table XLI. 



Table XLI. — Survey of the Effect of Selection in Pure Lines 

(The dark-faced figures indicate mean weights in eg.; the light-face figures designate 
respective numbers of seeds.) {From Johannsen) 









Weight in 


eg. of the mother beans 






Mean 


The pure 






















weig 


hts 
he 


lines 






















of t 




20 


30 




40 


5C 




60 




70 


lines 


I 


















63.1 


. . .. 6 


4.9 


91 64.2 


145 


II 












57.2 


86 


54.9 


195 


56.5 


120 i 


5.5 


74 56.8 


475 


III 
















56.4 


144 


56.6 


40 e 


4.4 


98 55.4 


282 


IV 














• 


54.2 


32 


53.6 


163 i 


6.6 ] 


12 64.8 


307 


V 












52.8 


107 


49.2 


29 




t 


0.2 ] 


19 61.2 


255 


VI 








53.5 


20 


50.8 


111 






42.6 


10 




60.6 


141 


VII 


45 


9 


16 






49.5 


262 






48.2 


27 






49.2 


305 


VIII 








49.0 


20 


49.1 


119 


47.5 


20 










48.9 


159 


IX 








48. 5 


117 






47.9 


124 










48.2 


211 


X 








42.1 


28 


46.7 


412 


46.9 


93 










46.6 


533 


XI 








45.2 


114 


45.4 


217 


46.2 


87 










45.5 


418 


XII 


49 


6 


14 










45.1 


42 


44.0 


27 






46.5 


83 


XIII 








47.5 


93 


45.0 


219 


45.1 


205 


45.8 


95 






45.4 


712 


XIV 








45.4 


21 


46.9 


51 






42.8 


34 






46.3 


106 


XV 


46 


9 


18 










44.6 


131 


45.0 


39 






46.0 


188 


XVI 








45.9 


147 


44.1 


90 


41.0 


36 










44.6 


273 


XVII 


44 





78 






42.4 


217 














42.8 


295 


XVIII 


41 





54 


40.7 


203 


40.8 


100 














40.8 


357 


XIX 








35.8 


72 


34.8 


147 














35.1 


219 


I-XIX 


44.0 


180 


44.3 


835 


46.1 


2238 


49.0 


1138 


51.9 


609 < 


S6.1 ' 


194 47.9 


5494 



254 



GENETICS IN RELATION TO AGRICULTURE 



The above analysis not only demonstrates that Johannsen's material 
was a mixture of different "weight types" but it also gives striking proof 
that selection within a single pure line has no effect. Johannsen points 
out that in certain lines (/, X, XI) there seems to be a slight effect but 
that in others (VI, IX, XII, etc.,) an opposite tendency appears; while 
still others (//, ///, 7/77). are irregular. Generally speaking then no 
effect of selection is seen for there is no significant difference between the 
means of the several groups in each pure line. The apparent indications 
of selection effects are merely fortuitous variations. In each of these 
lines, therefore, the offspring of plus and minus variants exhibit complete 
regression to the mean of the particular line. In short, individual varia- 
tions were not inherited, only the characteristic modifiability of the particu- 
lar line was inherited. 

Johannsen did not rest here but continued to test his pure lines of 
beans during successive years. He found a certain amount of seasonal 
fluctuation in the range of variation and in the variation constants, yet 
each pure line maintained its own individuality as indicated by the varia- 
tion in weight of beans produced. And this maintenance of entity was 
accomplished in spite of repeated selections of smallest and largest beans 
so that each year every pure line was represented by two lots of plants, a 
"plus strain" grown from the largest beans and a "minus strain" grown 
from the smallest beans. Complete failure of such repeated selection to 
cause significant change in the mean weight of either strain was observed 
in each pure line. As illustrations the data on Lines I and XIX are 
presented in Tables XLII and XLIII. 

From these data it is evident that six years of selection of plus and 
minus strains within Line I produced no permanent departure in either 
direction. In fact the last column (B-A) actually shows an inverse effect 
during three of the six years. Moreover, if the average of the means for 
the six years in both strains be compared this conclusion is verified. 



Table XLII. — Selection-effect Duking Six Generations in Line I op the 
Princess Beans. {From Johannsen) 



Harvest 
years 


Total 
number 

of 
beans 


Mean weight of 
mother beans of 
the select strains 


Differ- 
ence 
b - a 


Mean weight of progeny seeds of 
select strains 


Difference 
B - A 




a-minus 


6-plus 


A-minus 


B-plus 




1902 


145 


60 


70 


10 


63.15±1.02 


64.85 + 0.76 


+ 1.70 + 1.27 


1903 


252 


55 


80 


25 


75.19 + 1.01 


70.88±0.89 


-4.31 + 1.35 


1904 


711 


50 


87 


37 


54.59 + 0.44 


56.68±0.36 


+2.09 + 0.57 


1905 


654 


43 


73 


40 


63.55 + 0.56 


63.64 + 0.41 


+0.09±0.69 


1906 


384 


46 


84 


38 


74.38 + 0.81 


73.00 + 0.72 


-1.38 + 1.08 


1907 


379 


56 


81 


25 


69.07 + 0.79 


67.66 + 0.75 


-1.41 + 1.09 



PURE LINES 



255 



Table XLIII. 



-Selection-effect During Six Generations of Line XIX of the 
Princess Beans. {From Johannsen) 



Harvest 


Total 
number 
of beans 


Mean weight of 
mother beans of 
the select strains 


Differ- 
ence 
b - a 


Mean weight of progeny seeds of 
select strains 


Difference 
B - A 




a-minus 


6-plus 


A-minus 


B-plu8 




1902 
1903 
1904 
1905 
1906 
1907 


219 
200 
590 
1,657 
1,367 
594 


30 
25 
31 

27 
30 
24 


40 
42 
43 
39 
46 
47 


10 
17 
12 
12 
16 
23 


35.83 + 0.44 
40.21 + 0.65 
31.39 + 0.29 
38.26 + 0.16 
37.92 + 0.22 
37.36 + 0.30 


34.78 + 0.38 
41.02 + 0.43 
32.64±0.21 
39.15 + 0.17 
39.87 + 0.16 
36.95 + 0.21 


-1.05 + 0.58 

+0.81 + 0.78 
+ 1.25±0.36 
+0.89±0.23 
+ 1.95±0.27 
-0.41+0.37 



The mean for the progeny of the plus strain is 66.12 + 0.28 and for the 
progeny of the minus strain, 66.66 + 0.33. The difference is -0.54 + 0.43 
(the probable error of the difference in all cases being found by taking 
the square root of the sum of the squares of the two probable errors). 
In Line I, therefore, there is no positive effect of selection; on the con- 
trary there would appear to be a slight inverse effect! 

Line XIX was characterized by beans of the least weight. The data for 
the results of six years of selection in plus and minus directions, particu- 
larly the difference between the progeny means (B-A), reveal somewhat 
larger fluctuations in the plus direction than in Line I but it will be noted 
that the probable errors of the differences are smaller, hence the validity 
is the more certain. Comparing the means of the means of the progeny 
seeds as before, for the plus strain we have 37.40 ±0.11 and for the 
minus strain, 38.83 + 0.15, the difference being +0.57+0.19, which is 
certainly small although in the plus direction. Now, if we compare the 
summaries of the data from these experiments, -0.54 and +0.57, we 
are forced to conclude that selection was without effect in these pure lines. 
Finally Johannsen conducted similar experiments with the Princess 
beans, using the characters, length and breadth. He came to the same 
general conclusion, to wit, that he found no trace whatever of selection 
effect within pure Hues and that the variations in pure line individuals are 
merely fortuitous modifications and are not inherited. 

Conditions Necessary for the Existence of Pure Lines.— Johannsen 
defined a pure line as the progeny of a single self-fertilized individual of 
homogeneous factorial composition. Unless mutation takes place none of 
the descendants of such an individual can differ from the parent in their 
genetic factors. Two important conditions are imposed by this 
definition, viz., homozygosity and self-fertilization. The latter of these 
is the more fundamental inasmuch as it is mathematically demonstrable 
that self-fertilization, if continued generation after generation, leads 



256 GENETICS IN RELATION TO AGRICULTURE 

rapidly toward a homozygous condition in all descendants. Thus, 
Jennings shows that in the case of the original cross, A A by aa giving all 
Aa, if thereafter all breeding is by self-fertilization, then, after n genera- 
tions, the proportions of different genotypes in the population may be 
calculated by the following formulae: 



AA = 

Aa = 
aa = 



2n+ 1 

2"' 
2" - 1 

2n+ 1 • 



Therefore, within six self-fertilized generations after a cross involving a 
single pair of factors, the proportion of homozygous individuals in the 
population for one or the other of the two factors will be 98.4 per cent. 
Hence it is clear that, even though many genetic factors are concerned, 
as is undoubtedly the case in any crop plant or domestic animal, yet in 
those species where self-fertilization is the method of reproduction, the 
fundamental condition necessary to the existence of pure lines is met. 
Although by definition every pure line is a genotype, yet every genotype 
is not a pure line, for any heterozygote belongs to some genotype whereas 
a pure line is necessarily homozygous. Upon the basis of Johannsen's 
definition, it would be impossible to obtain pure lines from obligatory 
allogamous species, to which class belong all domestic animals and 
certain cultivated plants. However, it is clear that continual inbreed- 
ing in such organisms would tend to produce a homozygous genetic 
composition. 

Isolation of Pure Lines from Mixed Populations. — In order to 
obtain pure lines from mixed populations the method employed will de- 
pend upon the method of reproduction of the organism. In autogamous 
species the method adopted by Johannsen in working with beans is 
adequate. The individual plant being capable of reproducing the species 
through self-fertilization and incapable of natural cross-fertilization, 
it is only necessary to isolate the progeny of single individuals to establish 
pure lines. However, in supposedly autogamous species natural hybrids 
sometimes occur. Hence in critical work it is always advisable to pro- 
tect the flowers even of autogamous plants. In dealing with allogamous 
species, in which it is necessary to mate two individuals, when starting 
with a mixed population of unknown genetic factors the original selections 
must be made on the basis of phenotypic similarity. With domestic 
animals the repetition of such selection for a large number of generations 
has produced the "pure" or pedigreed breeds, which approximate more 
or less closely to pure lines and hence should be expected to breed fairly 



PURE LINES 257 

true to type. With plants the method of procechire depends upon the 
details of reproduction in the species under consideration. For example, 
corn is naturally cross-fertilized but is also self-fertile, while the common 
sunflower is self-sterile and so must always be cross-fertilized. With 
such plants as the sunflower, then, the procedure will be as with animals 
and the length of time required to produce approximately pure lines will 
depend upon three things: (1) the number of genetic factors for which 
each of the selected individuals is heterozygous; (2) the number of genetic 
factors with respect to which the two selected individuals differ; (3) the 
number of chromosomes in the species. The specific chromsome number 
is an important consideration because of its direct relation to the number 
of linked character groups or in other words to the possible number of 
freely assorting pairs of factors. Sufficient has been said concerning the 
comparative ease of isolating pure lines from populations of autogamous 
species and the relative difficulty of obtaining pure lines from allogamous 
species to make it clear that the material under consideration is of the 
highest importance in all critical discussions of the effect of selection 
within pure lines. Finally, it is to be noted that a vegetatively pro- 
pagated phenotype may or may not be a pure line according to its 
genetic constitution. A group of individuals thus propagated is known 
as a clone. In strictly allogamous species a clone would hardly ever be 
homozygous. 

The Effect of Selection Within Pure Lines. — There is now con- 
siderable evidence in support of the theory that selection within a pure 
line is without effect. This evidence comes from the results of practical 
breeding as well as scientific investigations of certain autogamous species 
of plants, such as wheat, oats and barley; also from thoroughgoing re- 
search on a few allogamous species, especially on certain insects and pro- 
tozoa, particularly paramecia. The constant maintenance of head type in 
wheat is strikingly portrayed in Fig. 106, which shows two heads from each 
of four varieties which were first isolated by Louis de Vilmorin between 
1836 and 1856. The plants according to Vilmorin were found to be 
identical in all respects "although separated by an interval of 50 years 
during which annual selection had been continued. This fixity is shown 
not only in the characters of the ear but also in all the other characters 
of the plant even that of precocity, which would appear to be most 
dependent on climate." The use of this case as evidence in support 
of the pure-line theory has been criticised upon the ground that the selec- 
tion practised had for its purpose the preservation rather than the altera- 
tion of the type. But from the experience of many investigators and 
breeders we may safely conclude that within true pure lines selection is 
without effect on the type unless mutations occur. After subjecting a 
variety of barley known as Glorup to plus and minus selection for eight 

17 



•258 



OEXETICS I\ UKl.ATTOX TO ACincn.Tr Ix'K 



s2;oniM;Uions, \\\c charartcv uiuUm' observatiiMi biMui:; dogro(> of mealiness 
of the kernel i^Sehartigkeit), .lohannsen eonehuled ihat (he seleelion 
had prodneeil no etVeel. ^Moreover (he Swedish plan(-breeiling s(a(ion 
at Svalof has been guided for years by (he knowledge (hat (heir pedigree 
eul lures, i.e., pure lines, were not ehangeil by seleetion. A sinnlar eon- 
olusion was reaehed by Tower after four to ten generations of rigorous 
selection of albinie indi\iduals in three dilVerent attenijits to establish 
an albinie raee from a stable raee (^pedigree material) of the C\ilorado 
potato beetle {lA'ptinofa)\^a (hccniJiiicata). The history of these three 




Fig. 100. — Four pure linos of wlioat which have boon grown by Vilniorin for 50 years. 
Tho orisxinal spocimon iu the seed museiun is shown on the left iu each case. The close 
siniihuity of the pairs of heads indicates that pure lines remain constant indefinitely. 
(.\j'tcr Iltiijidooni.) 



experiments are shown at A, B and (' in Fig. 107. The small black 
polygons show for each generation the imlividuals selected to become 
the parents of the next generation. It will be noted that neither the 
range nor the mode of the population is permanently shifted in the 
direction of the selection. Thus we find that in races or varieties which 
are constant (homozygous) selection has no etl'ect unless mutations occur. 
Vaiious evidence has been brought forward to show that the principle 
does not hold for all organisms. But in all such cases among sexually 
propagated species we may assume that the material used was hetero- 
zygous for certain factors. Such has been shown already to be a satisfac- 
tory explanation of Castle's results in selecting for phis and minus 
strains in the hooded rats which is one of the cases originally advanced 
as evidence against the pure line theory. 



I'nia-: fJNf'js 



250 



Significance of the Pure Line Theory in Breeding. The, quoHtjon 
UiijH urisfs: Wow does the pun; lino (Jioory explain tlic fact that man ha.s 
wroiij.'lii f»ofoijnr] changes in fJonicsticated animals and plants by Holec- 



" Normal' Uarnje of Variation 



"Norrnal" KariKc of Variation 



Mode 



Mode 




I'k;. 107. — Diagrammatic represftntation of results of three experiments in selecting 
beetles in an effort to create an albinic strain from a pure strain, (from Twjjer.) 

tion? It is well known that as a rule a mixed population coasists of a 
n limber (probably quite large) of distinct biotypes and that in autoga- 
mous species these biotypes are pure lines to begin with, while in alloga- 
mous species it is only by continued inten-sive selection that existing 



260 



GENETICS IN RELATION TO AGRICULTURE 



biotypes can be differentiated from one another so that they "breed 
true." How these distinct biotypes originate will be considered in the 
following chapter, the fact that they exist is the chief consideration here. 
The effect of "mass" selection in causing temporary changes in heteroge- 
neous varieties of plants and races of animals is easily understood by 
the aid of the diagram shown in Fig. 108. The area within the la,rge 
curve represents a mixed population or phenotypically similar group 
containing a number of distinct genotypes indicated by the small curves 
A-Z. Every genotype has its own variation curve and is distinct from 
each of the others, but they intergrade with each other so completely 
that the population appears as an entity. Now if one should select 
individuals from either extreme of the population, say at 90 or 70, it is 
clear that such individuals might belong to any one of four or five geno- 




JOM 62 C3 64 as eOWOii 687071 727374 



Fig. 108. — Schematic diagiam showing the relation of a population to the biotypes 
composing it, or of a phenotype, to the genotypes or pure lines within it. (After Lang 
from Goldschmidt.) 



types. If selection in the same direction were continued a strain would 
be established with a mode distinct from the mode of the original popu- 
lation. These strains could be maintained by continual selection 
and in time a single genotype might be isolated when selection would 
be said to have changed the type permanently. But selection changed 
nothing — it only isolated a certain genotype or genotypes from the origi- 
nal mixture. Tower's results in selecting for the purpose of creating 
albinic and melanic strains of beetles as illustrated in Fig. 109 may be 
explained in this way. The original population shown at A consisted 
of a number of distinct biotypes. By the isolation of several extreme 
variants Tower separated plus and minus strains which he was able 
to maintain for eight generations by practising intensive selection. In 
the eighth generation he divided each population in half, continuing in- 
tensive selection with one portion and stopping all selection in the other. 
By this method he was able to maintain the plus and minus strains and at 
the same time to observe that in the ninth generation the mode of the 



PURE LINES 



201 



progeny of the unselected eighth generation population lay much nearer 
to the mode of the original population. Within three generations the 
unselected strains had moved back to the mode of the species. Now 
it is to be remembered that Tower was dealing with an obligatory allo- 
gamous species. Moreover, what is now known concerning body 
pigmentation in Drosophila makes it altogether likely that quite a large 



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Fia. 109. — Diagrammatic representation of the results obtained by the creation of 
albinic and melanic strains from a mixed population of beetles. {From Tower.) 

number of genetic factors are concerned in the degree of pigmentation 
of these beetles. Hence selection of phenotypes for a number of genera- 
tions did not isolate genotypes, i.e., the plus and minus strains were not 
homozygous. As this is an allogamous species undoubtedly most of the 
individuals in the original population were heterozygous for many factors. 
Furthermore, Tower did not select single pairs but always took several 



262 GENETICS IN RELATION TO AGRICULTURE 

pairs as parents for each generation. While the selection of similarly 
pigmented individuals would tend gradually toward a homozygous 
condition,with respect to the specific factors conditioning pigmentation, 
yet it is altogether likely that under the conditions of the experiment 
a considerable degree of heterozygosity was maintained. In other words 
the selection practised did not isolate pure lines, the plus and minus 
strains did not become homozygous. Much of the work done in the 
past in ameliorating animals and plants has been by this method of 
selecting phenotypes but not genotypes, which accounts in part for the 
frequent necessity of continuous selection in maintaining improved strains 
or breeds. In reviewing the development of plant breeding we shall note 
certain cases of early recognition of the effects of genotypic selection, 
a principle which is now accepted as fundamental in all breeding 
operations. 



CHAPTER XIV 
MUTATIONS 

Baiir's third category of variations comprises all inheritable changes 
due to causes other than segregation and recombination of genetic 
factors. Although comparatively little is known concerning the specific 
causes of mutations, yet it is possible to distinguish between two general 
classes of such inheritable variations according to the nature of the 
genetic units involved. These classes are (1) alterations in genetic 
factors, and (2) deviations in the number of chromosomes. We designate 
the first group as factor mutations and the second as chromosome aber- 
rations. Since the first group is of vastly greater importance to agri- 
culture than the second, we shall consider the latter very briefly before 
engaging in discussion of the former, which we deem worthy of recognition 
as mutations in the strict sense. 

Chromosome Aberrations. — By the aid of cytology it has been demon- 
strated that inheritable changes are occasionally induced, in plants at 
least, by irregularities in the behavior of the chromosomes during mitosis 
or meiosis, such that certain germ cells contain fewer or more chromo- 
somes than the number typical of the species. Aberrant forms in 
several plant families are now known to differ from the parent species 
in chromosome number. Some have only a single chromosome more or 
less than the parent, while a few are known in which the original number 
is doubled. It is possible that aberrations occur involving all combina- 
tions of numbers between these two extremes. In various forms of La- 
marck's evening primrose {(Enothera lamarckiana) , whose typical number 
is 14, according to Gates the following aberrant numbers have been 
reported— 15, 20, 21, 22, 23, 27, 28, 29, 30. Aberrations involving the 
doubling of the number of chromosomes typical of the species is known 
as tetraploidy because there are four times the haploid number typical 
of the parent. Occasionally aberrations or hybridization between diploid 
and tetraploid forms result in triploidy. 

There is a limited amount of evidence which indicates that groups of 
species have arisen by progressive alterations in chromosome number. 
Thus in Drosophila, Metz has found ten species in which the 
chromosome numbers range from 6 to 12 and the larger numbers 
appear to have arisen by subdivision of the large dumbbell-shaped 
chromosomes found in the species having smaller numbers. Evidence 

263 



264 GENETICS IN RELATION TO AGRICULTURE 



1 



that doubling of the chromosome number may occur during somato- 
genesis has been found by Farmer and Digby in the interesting hybrid, 
Primula kewensis. The original plant, which was sterile, "had 18 and 9 
chromosomes in its premeiotic and postmeiotic nuclei respectively," 
but in the fertile plants which were propagated asexually from it, as 
well as in similar fertile hybrids which were produced in later experi- 
ments, the diploid and haploid numbers were 36 and 18 respectively. 
Having found by means of careful measurements of the chromosomes in 
the two forms that the nuclei in both forms contain the same volume of 
chromatin, the authors conclude that the increase in number may be 
attributed to transverse fission of the 18 larger chromosomes and not to 
the fusion of two nuclei. 

From a study of chromosomal dimensions in relation to phylogeny, 
Meek "arrived at the conclusion that the widths of chromosomes are 
successively greater in higher zoological phyla, and that this dimension 
is constant for very large groups of animals." But Farmer and Digby 
have shown that such a conclusion is without foundation since "closely 
related forms may possess chromosomes differing widely in shape and 
size and character." Hence they conclude "that phylogenetic affinity 
is not, necessarily, correlated with chromosome width." They also 
point out that "unfortunately we know practically nothing about the 
phylogeny of the chromosomes. No convincing hypothesis has been 
put forward to explain how these remarkable bodies have become 
organized, nor how their peculiarities have either been brought into 
existence or are kept so true for a given species." However, we are 
reminded by Glaser that chromatin is present in bacteria though not in 
the form of a nucleus and it may not be too much to hope that cytology 
may yet discover the principal stages in the development of the chromo- 
somes and establish such correlation as may exist between this develop- 
ment and organic evolution. Certainly extended investigations of 
chromosome numbers must be made before chromosome aberrations 
can be considered an important factor in evolution. Except that 
certain chromosome aberrations, such as tetraploidy causing gigantism, 
might be of economic value, in general this class of mutations is of minor 
importance in breeding. 

Factor mutations, on the other hand, are of prime importance and of 
general occurrence. Factor mutations have appeared in controlled 
cultures of many animals and plants and the character differences con- 
ditioned by them are as a rule such as distinguish varieties of a single 
species. Moreover, varietal characters are Mendelizing characters 
in the narrow sense and the existence of simple Mendelian phenomena 
among all classes of sexually propagated organisms proves that factor 
mutations are of general occurrence. Although it is probable that every 



MUTATIONS 



265 



factor mutation has a certain effect upon every character in the organism, 
yet the visible effects of some factor differences are restricted to a single 
character. According to their visible effects, therefore, we recognize two 
classes of factor mutations: (1) those conditioning apparently only single 
characters; (2) those having a visible manifold effect on the soma. Cases 




Fig. 110. — A seedling of the oak-like walnut (left) and of the California black walnut, 

the parent species (right.) 

involving mutations of the second class are known in several species of 
animals and plants. An interesting example is the oak-like walnut, 
Juglans calif ornica var. quercina, which appears to differ from the parent 
species by a single factor difference, Fig. 110. But this variety is distinct 
from the species type in nearly all gross morphological characters. 



266 GENETICS IN RELATION TO AGRICULTURE 

The Nature and Causes of Factor Mutations. — Our knowledge of 
genetic factors is entirely of an inferential sort and it is probable that 
these ultimate hereditary units are no more likely to be objectively 
perceived than are the atoms of which all matter is generally believed 
to be composed. But our present understanding of biochemistry and the 
chromosome mechanism of heredity leaves no room for doubt concerning 
the theoretical nature of these factors. Living protoplasm is generally 
considered as composed of very complex organic compounds. The 
phenomena of stereochemistry, especially the substitutional or cyclic 
changes which occur within various compounds under proper con- 
ditions, suggest that similar compensatory relations exist between 
the substances composing the living cell. Yet cytological observations 
indicate that the chromatin is the only permanent constituent of the 
nucleus and that the chromosomes are unaffected by the regular physio- 
logical processes of metabolism, growth and reaction to stimuli even 
though they play a very definite role in all these activities. As was 
explained in Chapter IV, the chromosomes are linear series of loci whereat 
are located specific factors. According to the multiple allelomorph 
hypothesis more than one factor may exist at a given locus. Since the 
chromosomes appear to consist of the only permanent substance in the 
nucleus, it is conceivable that at each locus there exists a unique chemical 
system; yet it is not unreasonable to suppose that occasionally substi- 
tutional changes similar to those known to take place in less complex 
organic compounds may occur here. 

The contributions of Eeichert on the specificity of proteins and 
carbohydrates as a basis for the classification of animals and plants are 
based on the fact that such substances as serum albumin, hemoglobin, 
glycogen and starch exist in stereoisomeric forms. That is, ''each kind 
of substance may exist in a number of forms, all of which forms have the 
same molecular formula and the same fundamental properties in common, 
but each in accordance with- variations in intramolecular configuration 
has certain individualities which distinguish it from others. ... It has 
been found that the number of possible forms of each substance is de- 
pendent upon the possible number of variations of the arrangements of 
the molecular components in the three dimensions of space, or, in other 
words, of variations of molecular configuration, the possible number in 
case of each substance being capable of mathematical determination. 
Thus, we find that serum albumin may exist in as many as a thousand 
million forms. Hemoglobin, the red coloring matter of vertebrate blood, 
is a far more complex carbon compound than serum albumin, and theoret- 
ically may exist in forms whose number is beyond human conception, 
running into millions of millions. The same is true of starch." Having 
in mind this complex molecular structure of protoplasmic constituents 



MUTATIONS 267 

and the phenomenon of substitutional changes of atoms or radicals 
by which such complex compounds are transformed, we can express a 
conception of the nature of factor mutations. 

To be specific let us suppose that some unusual condition occurs in a 
certain germ cell of a normal female Drosophila such that a single atom 
in each of the very complex molecules of the substance unique for the 
locus W in the X-chromosome changes place with a different atom in 
the surrounding nucleoplasm — the substance unique for the locus W 
is no longer capable of conditioning the laying down of red pigment in 
the eyes and, if the affected ovum is fertihzed by a F-bearing sperm, 
a white-eyed male appears, the result, as we say, of a factor mutation. 
This conception of factor mutations is useful as a basis for the multiple 
allelomorph hypothesis. In order to explain how two or more factors 
may have the same locus in a chromosome, it is only necessary to assume 
as possible the substitution of two or more different atoms or radicals 
in the molecule of the complex organic substance unique for the given 
locus by other atoms or radicals in the nucleoplasm. 

Factors are relatively stable entities however. It has been shown al- 
ready that any organism must possess thousands of factors, yet mutations 
are comparatively rare even in Drosophila. These facts are rather 
difficult to harmonize with our conception of the nature of factor muta- 
tions. If substitutions of atoms or radicals occur why do they not take 
place more frequently? Such questions must remain obscure until we 
know something about the chemical constitution of the hereditary factors. 
Only then can we expect to understand clearly the nature of the altera- 
tions which occasionally are made in them. 

In this connection the behavior of factor mutations in inheritance is of 
decided interest. As a rule they are recessive to their normal allelo- 
morphs and for some time they were thought to be due to the loss of 
factors, this idea being associated with the presence and absence hypothe- 
sis. But on rare occasions dominant mutations have appeared. Among 
150 mutations from the normal type of Drosophila ampelophila several, 
such as bar eye, dark streak on thorax, abnormal abdomen and CIII, a 
factor which modifies eosin eye color, are dominant over their respective 
allelomorphs. A few other mutant characters have been found to be 
dominant, such as hornlessness in cattle and red buds in the evening prifti- 
rose (CEnothera rubricalyx), but the great majority are recessive as is 
indicated by the ratio in F2 from crosses between mutants and normal 
individuals. The condition in Fi by no means always indicates complete 
dominance of the normal character. Hence it is clear that whatever the 
nature of the mutation-producing chemical change may be, as a rule it is 
either completely subordinate to the normal condition or else it merely 
modifies the effect of the normal state in heterozygous individuals, making 



268 GENETICS IN RELATION TO AGRICULTURE 

its own distinctive manifestation in one-fourth of the progeny of such 
individuals. 

When we enquire as to what are the particular conditions or specific 
antecedent events that make possible or cause the assumed substitution 
of atoms or radicals, we find ourselves again confronted by an almost 
total lack of knowledge. One thing is certain however, namely, that 
factor mutations are not fortuitous in occurrence, because, if they were 
the outcome of wholly indeterminate series of events, they would be 
as likely to occur in one species or race as in another at a given time 
and with the same relative frequency under all conditions, but such is 
not the case. On the contrary, certain species appear to be much more 
prolific in factor mutations than others and, as stated in Chapter II, 
it would appear that inheritable variations can be induced under 
controlled environmental conditions in pedigree strains that have 
bred true for a number of generations. Furthermore, even though 
our knowledge of the occurrence of factor mutations were so meager 
as to furnish no basis for reasoning and even though future observations 
of the same might seem to indicate that they are fortuitous, we should 
still be justified in assuming the existence of specific causes for factor 
mutations. It has been clearly shown by Pearl that, while natural 
phenomena are the result of long series of antecedent events or con- 
ditions, yet these are not all of equal determinative value; but rather 
that there are always specific causes which are few in number, immediate 
in time and large in relative quantitative effect. It does not seem 
necessary to present here the course of reasoning on which this con- 
clusion rests. The important thing for agriculture is the fact that 
factor mutations are caused and the possibility that some of the deter- 
minative antecedent conditions are external to organisms, i.e., that 
they exist in the environment and are controllable by man. The prob- 
lem of the exact nature of factor mutations is only a phase of the 
general problem of the nature of living protoplasm, the solution of which 
is one of the ultimate aims of biology. But it is possible at least that 
experimental research may reveal methods by which factor mutations 
can be induced in both plants and animals. 

Factor Mutations Both Germinal and Somatic. — Factor mutations 
appear to occur in undifferentiated cells, the germ plasm or embry- 
onic tissue in animals and either the germ cells or any meristematic 
tissue in plants. Occasional discontinuous variations are found in 
animals which might seem at first to be due to factor mutations in the 
developing soma. But most of these abnormalities are more satis- 
factorily explained in other ways. Thus, gynandromorphism, or the 
condition of having one side of the body male and the other female, 
has been reported in insects more than a thousand times according to 



MUTATIONS 269 

Morgan. Without doubt it is caused by some irregularity in the proc- 
ess of fertilization. Homeosis, or the replacement of one organ by 
another, is known to have followed mutilation. Examples of the modi- 
fication of characters by environmental conditions are given in Chapter 
II. There are many similar variations in animals, none of which are 
hereditary. However, we shall again refer to the possibility of somatic 
mutations in animals. 

There is no direct evidence as to the cytological time of factor muta- 
tions, but the stage in the germ cell cycle of animals at which factor 
mutations are most likely to occur would seem to be shortly before 
or during the process of maturation. This is indicated by the sporadic 
appearance of mutants. The first observed mutation in Drosophila 
ampelophila was white eyes, which were found in a few males among 
several hundred individuals in a pedigreed red-eyed race. Similarly 
with other sex-linked mutant characters that have been observed in this 
species, they have appeared either singly or at most in a few individuals. 
Had these mutations occurred at an earlier stage in the germ cell cycle, 
more gametes would have been affected and more mutant individuals 
would have been found. Obviously the length of time that must elapse 
before a factor mutation can manifest its existence depends upon two 
things in addition to the stage in the germ cell cycle at which it occurred : 
(1) its relation to its normal allelomorph, i.e., whether it is dominant or 
recessive; (2), its relation to sex determination, i.e., whether it is sex- 
linked or not. A mutation from W to win an X-chromosome of a normal 
male Drosophila would have produced a heterozygous red-eyed female in 
the next generation and no white-eyed flies whatever. One-fourth of the 
progeny of such a female would in turn be white-eyed if she mated with 
a normal male. Similarly with any non-sex-linked recessive cha'racter 
which upon its first appearance in pedigree culture is found in more than 
a single individual the probable order of events is as follows. A muta- 
tion occurred in a single germ cell of a single individual, which mated 
with a normal individual, thus giving rise to one heterozygote among its 
progeny. This heterozygous individual mated with a normal individual, 
producing heterozygotes among one-half of their progeny. Finally some 
of these heterozygotes mated together and one-fourth of their progeny 
bore the recessive mutant character. 

It would seem, therefore, that factor mutations in animals occur in 
the germ cells shortly before or during maturation and the time of appear- 
ance of a mutant character depends upon the relation of the mutant 
factor to its normal allelomorph and whether or not it is contained in 
the sex chromosome. 

In plants factor mutations may occur in any meristematic tissue as 
well as in the germ cells. Observations on the occurrence of mutant 



270 



GENETICS IN RELATION TO AGRICULTURE 



seedlings indicate that, as in animals, germinal mutations usually occur 
just before or during the maturation process. The strongest evidence 
for this conclusion is the fact that, so far as known, new dominant char- 
acters appear first in only one or two individuals. The following cases 
illustrate this point. The red-leaved evening primrose, (Enoihera 
rubricalyx (Fig. 118) has been known to occur but once in all ffinothera 
cultures and then in a single plant. The red sunflower, Helianihus 
lenticiilaris coronatus, as reported by Cockerell, first appeared as a single 
plant which proved later to be a heterozygous 
dominant. A purple-leaved mutation in hemp, 
Cannabis saliva, is reported by Dewey to have 
first appeared in two pistillate plants in a closely 
inbred strain of normal green plants. Had these 
mutations occurred at some preliminary stage 
in germ-cell formation, the change in chemical 
constitution would have been transmitted to 
several or many gametes and a considerable 
number of individuals would have appeared 
instead of only one or two. 

Factor mutations in meristematic cells, or 
vegetative mutations, as distinguished from 
those originating in the germ cells, give rise to 
simple bud sports or to chimeras according to the 
location of the mutating cell. A bud sport is a 
shoot or branch which differs genotypically in 
one or more characters from the remainder of the 
plant. Here the factor mutation must occur in 
one of the undifferentiated cells of the very 
young shoot. Just as in the case of factor 
mutations in germ cells, so in vegetative muta- 
tions the somatic effects range from single visible 
character differences to manifold effects in which 
many structural details are different. An example 
of bud sports in which the factor mutation 
induced a single character difference is shown in Fig. 111. The early 
gladiolus known as ''The Bride" is a white variety of Gladiolus colvillei, 
a red-flowered form, and doubtless originated from it as a seed or bud 
mutation. In 1915 there appeared in a row of ''The Bride" a single 
stalk bearing partly red and partly white flowers. That this grew from 
a corm which was an offshoot from a typical white-flowering corm is 
certain. Furthermore, that the mutation occurred very early in the 
development of this corm and not sometime during the growth of the 
flower stalk is proved by the following observation. In the autumn 




Fig. 111. — Bud sport 
from a white flowered 
gladiolus bearing red flow- 
ers on ore side of the stalk 
and showing one flower 
half red and half white; 
a sectorial chimera (see 
Chapter XXII). 



MUTATIONS 271 

following the discovery of the mutant stalk it was carefully lifted and 
the corm from which it grew was separated from the cluster of white- 
flowering corms. It was observed that there were smaller corms located 
very close to the mutant corm. The following spring one flower stalk 
bore red and white and the other only red flowers. In gladiolus the 
young corms push out from near the base of the old one. Hence the 
original mutant corm must have consisted partly of cells capable of 
producing red pigment in the flowers. That the cells having this altered 
chemical constitution comprised about one-half of the corm is indicated 
by the position of the red and white flowers on the stalk. This illustra- 
tion is hardly typical of all bud sports in that the mutation occurred too 
late in the development of the young shoot to change all the cells in the 
corm and so make all the flowers red. It was chosen first, because the 
mutant character is dominant/ which makes it certain that the sport 
was due to mutation rather than to segregation, and second, because 
it also illustrates the origin of chimeras. In many cases of discontinuous 
bud variation the entire shoot is affected. Cases of bud variation pre- 
sumably caused by factor mutations which condition manifold character 
differences are occasionally found in the citrous fruits. The so-called 
Australian Navel orange has undoubtedly arisen a number of times 
from the commerical variety, the Washington Navel orange, from which 
it differs in its propensity to rank vegetative growth combined with low 
productivity. Also the fruits are rough and of poor quahty. Numerous 
other distinct types of oranges and lemons have been discovered, usually 
as a single tree or merely a branch on a tree of the commonly cultivated 
variety (see Fig. 161). 

A chimera is a mixture of genotypically diverse tissues in the same 
shoot. The nature, categories and artificial production of chimeras and 
graft hybrids are discussed in Chapter XXII. Here it is only necessary 
to point out that as they occur in nature they undoubtedly owe their 
origin to factor mutations. In the red and white flowered gladiolus 
an entire shoot became composite in nature through a factor mutation 
in a meristematic cell very early in the development of the stem. If 
the mutation had occurred later on at just the right point in the vegetative 
cone, it might have produced a single red and white flower. This is 
apparently the manner of origin of the odd stripes on certain fruits 
such as the lemon shown in Plate II. In this case it is evident that 
mutations occurred in two different cells. In one case the factor change 
resulted in the laying down of yellow pigment of a deeper shade ("deep 
chrome," No. 176 of Ridgway's Color Standards) than that normal 
for the variety, which is lemon chrome. In the other case the mutation 

1 G. coluillei is a hybrid between G. cardinalis, which has bright scarlet flowers and 
G. tristis, which has white or yellowish flowers. 



272 GENETICS IN RELATION TO AGRICULTURE 

resulted in the production of some red pigment along with the yellow, 
thus causing the narrow sector of deep orange chrome (Ridgway, No. 
llh). That each of these changes occurred in a single cell is indicated 
by the fact that the differently colored sectors are sharply defined through- 
out and that the extremities of the orange red sector are extremely 
narrow. J. B. S. Norton reports the origin of a color chimera in the Acme 
tomato in which a branch of lighter green foliage appeared and the lighter 
colored tissue could be traced down the stem to a point where it had ap- 
parently originated in a single cell. Expanding as the stem grew, first a 
portion of a leaf was involved and finally an entire bud was included, thus 
giving rise to the sport branch. Undoubtedly this is the usual manner of 
origin of natural chimeras. 

We have examined several typical cases of factor mutations in animals 
and plants. From this evidence it is clear that factor mutations occur 
in undifferentiated cells — the germ cells in animals and either the germ 
cells or any meristematic cell in plants. There is, of course, no a priori 
reason why mutations should not occur in the somatic cells of animals. 
A fairly common meristic variation is the reduplication of repeated parts 
and it is possible that this departure from normal development is con- 
ditioned by a factor mutation. The discovery of a germinal mutation 
causing reduplication in animals would support this idea. Such a 
mutation has been discovered by Miss Hoge who reported a recessive 
factor for reduplication of the legs in the Drosophila. This possibility 
of somatic factor mutations in animals has little practical significance on 
account of the impossibility of propagating domestic animals asexually. 
It has considerable theoretical interest, however, in its possible bearing 
on the origin of certain diseases such as cancer. 

Vegetative Mutation Versus Somatic Segregation. — Since the ma- 
jority of bud sports are characterized by the replacement of a dominant 
with a recessive character, it is not strange that both bud sports and 
chimeras have been generally considered as due to ''somatic segregation" 
in heterozygous individuals. It is not yet known whether bud sports 
occur more frequently in heterozygous than in homozygous individuals. 
But this consideration is of less importance than the fact that somatic 
factor mutations do occur, which seems to be well established. To 
mention an illustrative case, Emerson has shown that the experiments 
of de Vries, Correns, Hartley, East and Hayes, and himself, ''all indicate 
that certain somatic variations are inherited in strictly Mendelian fashion. 
All these somatic variations consist in the appearance of self-colors on 
plants that are normally variegated in pattern. The fact that variegated 
plants occasionally throw both bud sports and seed sports with self- 
colors is not, in general, to be taken as an indication that the variegated 
plants in question are heterozygous. Such behavior seems to be insepa- 



o. 




Plate II. — Chimera in a Lemon. 

The broad sectoi of orange and the narrow sector of orange red were caused by factor mutations 
which occurred presumably in single cells at a very early stage in the development of the fruit. 



MUTATIONS 



273 



rably associated with variegation. Correns has pointed out that 
variegated Mirabihs plants cannot be considered mosaics of green and 
'chlorina' types due to heterozygosis, since they do not segregate into 
chlorina and green, but into variegated and green. The same reasoning 
apphes to variegation in the color of maize ears. Variegated-eared plants 
do not throw reds and whites, but reds and variegates. The conclusion 
seems irresistible that self-color occurring as a somatic variation is 
due to the change of a Mendelian factor for variegation into a factor for 
self-color. If this be granted, the behavior of these variations in later 
generations is a mere matter of simple Mendelian inheritance." 

If bud sports are caused by mutations and if most bud sports involve 
a change from the dominant to the recessive condition of a certain factor, 
it follows that the change in chemical constitution must affect both of the 




Fig. 112. — Bud sport and chimera in an ear of corn. This ear appeared in a field of 
white dent corn. The apparently white Ivernels, occupying about 3^ of the surface, were 
actually variegated, being marked with "fine red lines, or streaks, radiating from the caps 
down the sides of the kernels." (After Hartley.) 

duplex factors present in the somatic cell in order that the recessive 
character may appear. To those who think of mutations as fortuitous 
events, this may seem an obstacle to the conception that bud sports 
are the result of factor mutations. But from the point of view that 
factor mutations are caused, probably by some specific internal condition, 
it would seem most natural for the cause to have the same effect on both 
factors. Obviously this conception assumes that in such cases the specific 
cause, whatever it is, has the same potentiality in all parts of the nucleo- 
plasm, and there is no a priori logical objection to such an assumption. 
At the same time there is good evidence that mutations do sometimes 
occur in only one of a duplex pair of factors. Hartley reports "a remark- 
able ear (Fig. 112) occurring in a field of white dent corn which had 
for many years been grown as a reasonably pure corn, but which occa- 
sionally, as many white corns do, produced a red ear." But this ear was 
only partly red since about one-fifth of its surface was occupied by varie- 
gated grains which appear to be white in the picture. Hartley tested 
all the grains on this ear and found thai, the red grains produced 
a crop of 84 red ears and 86 pure white ears, while the variegated grains 

18 



274 GENETICS IN RELATION TO AGRICULTURE 

produced 39 variegated ears and 36 pure white ears, which is clearly 
a 1 : 1 ratio in each case. This proves that both types of grains were 
heterozygous for a dominant mutant factor and that both of the factor 
mutations occurred in only one member of a duplex pair of factors. Pre- 
sumably the mutation from white to variegated occurred first, and later 
the mutation from variegated to red in a cell so located that, as the shoot 
developed, only a portion of the ear was affected. 

There appears a very important obstacle to the conception of "somatic 
segregation" in that the mechanism of cell division is apparently one of 
the most nearly perfect and regular of natural systems and that the order- 
liness of procedure is especially notable in undifferentiated tissue, where 
bud sports and chimeras commonly originate. To assume that the oc- 
currence of self-colored flowers on variegated plants is due to chromosome 
aberrations in mitotic divisions is much less plausible than to explain such 
phenomena by assuming a simple factor difference as responsible for self- 
color and variegation, and that changes from one state to the other are 
possible under certain conditions. This is the only reasonable hypothesis 
by which to explain mutations from the recessive to the dominant condi- 
tion of a pair of factors, as we have seen in the case of Hartley's ear of red 
and variegated corn. Therefore, while chromosome aberrations are 
known to occur during mitosis and aberrant numbers of chromosomes 
have been found in senile and diseased tissues, yet, in general, bud sports 
and chimeras are satisfactorily explained on the basis of factor mutations ; 
whereas "somatic segregation" as the term has been used by Bateson, 
Gates and others implies the common occurrence of breaks in the 
mechanism of mitosis such as are not known to occur in normally 
functioning somatic cells. 

It should be remembered that horticultural literature contains nu- 
merous peculiar cases of discontinuous variation, many of which have been 
described or "explained" as "somatic segregations" resulting from 
hybridization. We believe that most of these cases can be explained 
much more reasonably in terms of factor mutations. But certain 
discontinuous variations in plants are undoubtedly the result of neither 
factor mutations nor chromosome aberrations in vegetative tissues. 
For example, persistent and deciduous calyx lobes are sometimes found 
on fruits of the same plant especially in the rose family. Tufts has 
described the occurrence of this phenomenon in the Le Conte pear and the 
Transcendant crab-apple as "somatic segregation," assuming that some 
sort of segregation-mechanism exists in the division of somatic cells. 
Data from the pear tree gave a ratio of 3.15 deciduous to 0.85 persistent 
lobes. But to assume irregularities in chromosome behavior such as 
would cause segregation preceding the formation of nearly one-fourth of 
the calyx lobes on the tree is unwarranted in view of the general regularity 



MUTATIONS 



275 



of the process of mitosis (see p. 60). It has been shown by Babcock 
and Lloyd that no special significance should be attached to the occur- 
rence of a ratio which, under the laws of simple sampling, could not occur 
oftener than once in 1,155,000,000,000,000 times, especially in view 
of the fact that these two varieties are presumably complex hybrids, and 
the persistency and deciduousness of the calyx lobes were variable in 
the parents. Hence to use the term somatic segregation in attempting 
to explain phenomena such as these is not only unwarranted but posi- 
tively misleading. 

The multifarious manifestations of dimorphism in plants arc, in 
general, the result of alternative expression of inherited characters rather 




Fig. 113. — Transition from..one form of leaf to another on the same branch in (a) Euca- 
lyptus globulus (b) Hedera helix. 

than alternative transmission of different factors. There are, to be sure, 
various cases of dimorphism within species, such as the different forms 
of flowers described by Darwin or the zygomorphic and peloric snap- 
dragon flowers, which usually do not appear together on the same 
plant and which exhibit alternative inheritance when crossed. But there 
are many species which bear different forms of branches, leaves, flowers 
or other organs on the same plant. Cook has described dimorphic 
branches in cotton, coffee, cacao, the Central American rubber tree and 
the banana, also dimorphic leaves in cotton, hibiscus, okra and allied 
genera. The open and cleistogamous flowers of the violet make a familiar 
example'of dimorphism in the same plant. In all these cases it appears 
that the individual plant contains all the factors conditioning the expres- 



276 GENETICS IN RELATION TO AGRICULTURE 

sion of the alternative forms. It seems reasonable then to explain the 
variations in somatic expression of the genetic factors present by internal 
changes of some sort. Frequently these variations appear as localized 
stages in ontogeny and it is possible that internal secretions (hormones) 
play a more important role in plant development than has been realized. 
The recent experiments of Loeb on Bryophyllum calycinun indicate not 
only the association and possible identity of root-forming and geotropic 
substances in this plant, but also that the leaves produce growth inhibit- 
ing substances which pass downward through the stem and which may 
accompany or may be identical with the root-forming hormones. Cook 
has shown that sometimes two extremely different forms of leaf occur on 
adjacent nodes but even such abrupt transitions might result from an 
internal reaction occurring in the interim between the development of 
the two successive leaves. Moreover, the transition from one leaf form 
to the other is frequently gradual as in the two series, each from a single 
branch, shown in Fig. 113. We conclude, therefore, that most cases of 
dimorphism in the same plant are not caused by factor mutations but 
rather that they should be classified with those cases of '' fixed dimorphism " 
so frequently found in insects and illustrated by the earwigs in Fig. 20. 
" Mutations" in the Evening Primroses. — Credit for directing atten- 
tion to suddenly appearing new forms of animals and plants both as 
material for origin of species and for improvement of domesticated races 
belongs to the Dutch botanist, Hugo de Vries. Other naturalists had 
previously noted such aberrant or anomalous organisms but without 
attaching much significance to them. Thus in the works of Darwin, 
especially in "The Origin of Species" and "Animals and Plants under 
Domestication," there are frequent references to aberrant individuals 
or sports and to curious groups of plants and animals like the niata 
cattle, which Darwin admits probably originated as definitely distinct 
individuals among the typical species group. Yet Darwin never con- 
sidered such aberrant individuals or groups as playing any significant 
role in evolution. On the other hand, de Vries became so convinced of 
the general occurrence and significance of suddenly appearing, heritable 
variations that he proposed a theory of evolution by mutation in which 
he applied Darwin's great principle of natural selection to these mutations 
as the general method of origin of species. The investigations which 
led him to this conviction extended over a period of nearly 20 years, dur- 
ing which time he brought under experimental cultivation some hundred 
species of plants that grow wild in Holland. They all exhibited more or 
less continuous variation; also he was able to isolate numerous strains 
which differed from the normal wild type with respect to some peculiar 
feature. But de Vries was searching for evidence of species "in the 
making" and he believed that by sufficient searching he should locate 



MUTATIONS 



277 



a species in which the transformation into new forms was proceeding 
on a scale large enough to make possible the direct observation of species 
formation. In none of the particular races that he collected did he 
observe profound discontinuous variations until in 1886 he discovered 
a feral group of large-flowered evening primroses {(Enothei'a lamarckiana) 
growing in a suburb of Amsterdam. They had escaped into an abandoned 
potato field from a nearby park. The source of this particular even- 
ing primrose has been traced by de Vries. About the middle of the 19th 




Fig. 114. — CEnolhera lamarckiana. {From a i^aiiitiiiy. See de Vries, Gruppenweist Art- 

bildung.) 

century seeds of CE. lamarckiana were imported into England from Texas. 
De Vries' race came from an estate near Hilversum, the seed having 
been obtained originally from an establishment in Erfurt, which de 
Vries thinks must have obtained their seed from England. It has never 
been found as an indigenous species either in Europe or America. This 
beautiful plant is much prized as an ornamental and is known to have 
escaped from cultivation in various places. 

"Lamarck's evening primrose is a stately plant, with a stout stem, attaining 
often a height of 1.6 meters and more (see Fig. 114). When not crowded the 
main stem is surrounded bj^ a large circle of smaller branches, growing upward 



278 



GENETICS IN RELATION TO AGRICULTURE 



from its base so as often to form a dense bush. These branches in their turn have 
numerous lateral branches. Most of them are crowned with flowers in summer, 
which regularly succeed each other, leaving behind them long spikes of young 
fruits. The flowers are large and of a bright yellow color, attracting immediate 
attention, even from a distance. They open toward evening, as the name in- 
dicates and are pollinated by bumblebees and moths. Contrary to their con- 
geners they are dependent on visiting insects for pollination. (E. biennis and 
CE. mvricata have their stigmas in immediate contact with the anthers within 




Fig. 115.^ — Leaf, flower bud, flower and essential organs of (Emothera rubrinervis (1-4) 
and CE. brevistylis (5-8). The specimen of brevistylis came from a red pigmented 
strain grown by Dr. R. R. Gates; the original brevistylis had no more red pigment than 
lainarckiana. 

the flower buds, and as the anthers open in the morning preceding the evening 
of the display of the petals, fecundation is usually accomplished before the insects 
are let in. But in CE. lamarckiana no such self-fertilization takes place. The 
stigmas are above the anthers in the bud, and as the style increases in length 
at the time of the opening of the corolla, they are elevated above the anthers 
and do not receive the pollen. Ordinarily the flowers remain sterile if not 
visited by insects or pollinated by myself, although rare instances of self-fertiliza- 
tion were seen .... Ordinarily biennial, it produces rosettes in the first, 
and stems in the second year" (de Vries). 

De Vries' original discovery consisted of the location of two aberrant 
groups among several thousand lamarckiana individuals. One of these 
new forms had smooth leaves and was named Icevifolia, the other had 



MUTATIONS 



279 



very short styles and was named hrevistylis. Each differed somewhat 
with respect to other characters as well (see Fig. 115) but were named for 
their most striking difference from the parent species. De Vries next 
proceeded to hunt for more new forms. By transplanting rosettes from 
the original locality to his garden he carefully compared them when they 
flowered the second year and saved guarded seed from the "mutants" 
so that he might test the inheritance of the new forms. He also gathered 
seed from two different lamarckiana plants in the open and these were 
the source of his lamarckiana "families." Of one of these families he 
raised several thousand plants from self-fertilized seed in each generation 
for seven generations and in each successive population he discovered 
a number of "mutants." This experiment is summarized in 
Table XLIV. 



Table XLIV.- 


-Pedigree of a 


Family of CE. Lamarckiana 


, 1886- 


-1899 


Generation 


Gigas 


Albida 


Oblonga 


Rubrinervis 


Lamarckiana Nanella 


Lata 


Scintillans 


I 










9 








II 










15,000 


5 


5 




III 








1 


10,000 


3 


3 




IV 


1 


15 


176 


8 


14,000 


60 


73 


1 


V 




25 


135 


20 


8,000 


49 


142 


6 


VI 




11 


29 


3 


1,800 


9 


5 


1 


VII 






9 





3,000 


11 






VIII 




5 


1 





1,700 


21 


1 




Total 


1 


56 


350 


32 


53,509 


158 


229 


8 



This summary shows that among a total of some 50,000 seedlings of 
self-fertilized lamarckiana plants seven different new forms appeared 
with varying frequency. The first two to be recognized and isolated 
for testing were nanella and lata (see Fig. 116). The dwarf variety, 
nanella, was also found blooming among the typical lamarckiana plants 
at the original station and these plants, like those dwarfs that appeared 
in the experimental garden, always bred true if self-fertilized. Lata 
on the other hand proved to be self-sterile because entirely devoid of 
viable pollen. When fertilized with larnarckiana pollen it produced 15 
to 20 per cent, of lata and the remainder lamarckiana. Later on de 
Vries discovered a hybrid strain of lata that produced some viable pollen 
and when self-fertilized these plants produced the same proportion of 
lata and lamarckiana progeny. For this reason de Vries considers it an 
inconstant species. In the third generation another new form appeared, 
which unlike nanella and lata was more robust than lamarckiana. It 



280 



GENETICS IN RELATION TO AGRICULTURE 



also had considerable more red pigment in the epidermis. This was 
especially marked in the marginal region of the sepals (see Fig. 118) 
and on the developing fruits. This form was named ruhrinervis and 
since it bred true when self-fertilized it was considered a well-defined 
"progressive" species, i.e., a species capable of maintaining itself in the 




Fig. 116. 



-CE. lamarckiana, lata and nanella. {From colored plate iv de Vries' Mutatiorts- 
theorie, vol. 2.) 



wild state. In the fourth generation four additional forms were found. 
One of these, gigas, appeared only once in de Vries' cultures but the 
one plant found in 1895 produced nearly 300 plants of gigas type from 
self-fertilized seed and the strain bred true thereafter. In recent years 
it has appeared a number of times in other strains of lamarckiana. 
This form is decidedly more robust than lamarckiana and the leaves are 



MUTATIONS 



281 



broader and of a deeper green, as is shown by Fig. 117. De Vries 
classified this form also as a progressive species. Albida and ohionga 
were classified as distinct but weak species, incapable of perpetuating 
themselves in a state of nature and hence were called "degressive." 
The seventh form, scintiUans, proved to be inconstant from the beginning, 
self-fertilized seed always producing some lamarckiana seedlings, as 




Fig. 117. — (E. lamarckiana {left) and gigas (right), flower stalks and rosettes. {From de 

Vries.) 



well as scintiUans, ohionga, lata and sometimes nanella. The two new 
forms found growing wild in 1886, Icevifolia and hrevistylis, never appeared 
among the experimental cultures but because they, like nanella, appeared 
to have lost something that characterized lamarckiana and to be dis- 
tinguished from it by one definite character, de Vries classified these 
together with nanella as "regressive" species or "retrograde" varieties 
at the same time pointing out that they possessed the qualifications of 



— nanella, loBvifolia, brevistylis. 



282 GENETICS IN RELATION TO AGRICULTURE 

elementary species. De Vries, therefore, classified the "mutants" as 
follows: 

I. Progressive species — gigas, ruhrinervis. 

II. Degressive species — albida, oblonga. 

III. Regressive species or 
retrograde varieties 

IV. Inconstant species — lata, scintillans. 

These were considered the important mutations although several 
others were recognized and given names but because of sterility or lack 
of space and time for growing them they were not preserved. 

The Mutation Theory of Evolution. — Based on the observations and 
experiments above reviewed, de Vries formulated a set of "laws of 
mutability" for the evening primroses which serve as an epitome of 
his theory of evolution. Omitting further discussion of the evidence for 
the present, the laws themselves are stated as follows : 

I. New elementary species appear suddenly without intermediate 
steps. 

II. New forms spring laterally from the main stem. 

III. New elementary species attain their full constancy at once. 

IV. Some of the new strains are evidently elementary species, 
while others are to be considered as varieties. 

V. The same new species may be produced in a large number of 
individuals. 

VI. Mutability is entirely independent of fluctuating variability. 
VII. The mutations take place in nearly all directions. To these 
an eighth must be added in order to complete the theory: 

VIII. Natural selection eliminates all unfit mutants originating in 
the wild. 

De Vries found many examples of the sudden origin of new forms in 
the history of domesticated animals and plants and pointed out various 
practical applications of his discovery, to some of which we shall have 
occasion to refer later. For the present it is necessary to give furtheJT 
consideration to the evidence in the case of the CEnothera "mutants" 
and to the interpretation thereof in order to arrive at a definite conception 
of the true nature of these aberrant forms. For this purpose it will 
be necessary to summarize in a general way the researches which have 
been made since de Vries' original work. 

The publication of "Die Mutationstheorie " aroused widespread 
interest and brought forth certain criticisms concerning the biological 
basis for de Vries' conclusions. The opponents of the theory assumed 
CEnothera lamarckiana to be of hybrid origin and pointed out that upon 
such a basis the so-called mutations are merely recombinations of ances- 



MUTATIONS 283 

tral characters. The controversy which arose over these questions 
stimulated investigation to such an extent that the CEnothera Uterature 
of the past decade woukl fill many volumes. These investigations have 
proceeded along three definite lines: (1) crossbreeding experiments, 
(2) cytological studies, (3) observations and collections in the field to- 
gether with pedigree cultures. The last of these can only be mentioned. 
See the recent reviews of Davis and Bartlett where further references 
are given. The results of the other two bear directly upon the problem 
of classifying de Vries' original "nuitations." 

Concerning the first method of investigating the genetics of Oenothera, 
there have been many crossbreeding experiments in both Europe and 
America. Until recently most of the data derived therefrom have been 
viewed as impossible of interpretation on a Mendelian basis. But 
since 1914 certain investigators have come to believe that breeding 
experiments with Q]]notheras have very little value as a means for exact 
genetical analysis unless complete germination of all viable seeds is 
assured. In that year Renner pointed out that seed sterility in the 
evening primroses may cause apparent noncomformity with Mendelian 
principles. Following up this suggestion with seed germination tests 
and breeding experiments in which all viable seeds were germinated, 
Davis came to the conclusion "that large proportions of Q^^nothera seeds 
sprout in the earth only after many weeks or even months and that this 
habit of delayed germination must have given in many of the cultures 
described in the CEnothera literature hardly more than glimpses of the 
genetical possibilities. We cannot feel certain that the records of any 
cultures of Oenothera so far reported are complete for their possible 
progeny, and consequently the ratios of classes described in breeding 
experiments and the percentages of 'mutants' calculated cannot be 
accepted as final in exact genetical work. We are not in a position even 
to guess what may be the change of front when exact data become avail- 
able. . . . Consequently we have at present in the ffinotheras no 
standard material of genetic purity with which forms under suspicion 
-may be confidently mated to determine by crossbreeding the uniformity 
of their viable gametes. Until such material is discovered we shall be 
working largely in the dark in our attempts to analyze the genotypic 
constitution of CEnotheras." The same author, is inclined to interpret 
data from his most recent CEnothera breeding experiment {biennis and 
franciscana) as giving "positive evidence of a segregation of factors in the 
F2 generation of a character to be expected in Mendelian inheritance." 
This inference is the more noteworthy inasmuch as in the past this author 
has not committed himself positively to a Mendelian interpretation of 
any particular data on the CEnotheras. That certain characters in this 
group are conditioned by specific genetic factors, seems to be generally 
accepted. For example, it is highly probable that the deeply pigmented 



284 



GENETICS IN RELATION TO AGRICULTURE 



character of (E. ruhicalyx is conditioned by one or two specific factors. 
This new form (see Fig. 118) was discovered by Gates in 1907, among a 
population of over 100 ruhrinervis plants grown from self-fertilized seed 
of ruhrinervis. The original ruhricalyx plant when self-fertilized produced 
12 plants, 11 ruhricalyx and 1 ruhrinervis, which would indicate that the 
original ruhricalyx plant was heterozygous for one or more factors for 
excessive production of anthocyanin and that ruhricalyx is dominant to 
ruhrinervis. Gates has raised ten generations of ruhrinervis (more than 




Fig. 118. — Flower bud and leaf of A, (Enothera ruhrinervis; B, CE. ruhricalyx. The 
deeper pigmentation of ruhricalyx is not confined to the bud and lower side of the leaves, 
but is also present_ in the stems. The rosette leaves also show more or less color on the 
vaiArih in j-uhricalyx. 

one pedigreed strain) and found it to breed true and he has one strain of 
ruhricalyx that has bred true for five years, but as yet there are no data 
on the results of a cross between them where the F% progeny were grown 
on a large scale and with controlled seed germination. However, in Fi 
ruhricalyx is dominant. Another O^^nothera character which is inherited 
in simple Mendelian fashion is the typical feature of hrevistylis (Fig. 115). 
Brevistylis is known to breed true when self-fertilized and the results 
of various crosses indicate that the short style is conditioned by a single 
factor, although it is not always completely recessive in Fi. Thus we 
find that, while most of the experimental breeding data on Oenotheras 
cannot be interpreted in terms of ordinary Mendelian concepts, neverthe- 



MUTATIONS 285 

less characters have been discovered that appear to be inherited according 
to simple Mendelian rules. We conclude, therefore, that some of the 
lamarckiana derivatives are the result of factor mutations. 

The cytological studies on ffinothera have yielded important infor- 
mation concerning the chromosome numbers of various species and "mu- 
tants." With reference to (E. lamarckiana and its derivatives especially 
the chromosome counts of Miss Lutz, Gates, Davis and others are of 
great interest. Lamarckiana has 14 chromosomes as have also most of 
the "mutants" which have been derived from it, but the sexually 
deficient and inconstant form, lata (see Fig. 116) has been found always 
to have 15 chromosomes. Furthermore, actual cases of a distribution 
of 6 + 8 chromosomes in the heterotypic division of pollen mother cells 
have been observed in lamarckiana and rubrinervis. It is safe to assume, 
therefore, that lata-\ike "mutants" result from the union of a gamete 
containing 8 chromosomes and one containing the normal number, 7. 
There is also good evidence that (E. gigas is the result of tetraploidy. 
Several different plants of this type have been found to contain 28 chromo- 
somes or thereabouts. However, there is a giant race of the Chinese 
primrose which has only 24 chromosomes, the number typical of the 
species, while another has 48 chromosomes. It seems then that gigan- 
tism is associated with tetraploidy but that it is not necessarily caused 
by an aberration in chromosome number. Thus we find that at least 
one, and perhaps another of the original lamarckiana derivatives are 
due to chromosome aberrations during meiosis. 

Of the nine original mutants we have now definitely classified two — 
hrevistylis as a factor mutation and lata, the result of a departure from 
normal chromosome number, and we have found that a third, gigas, 
exhibits an extreme chromosome aberration. What about the remain- 
ing six — Icevifolia, albida, ohlonga, rubrinervis, nanella and scintillansf 
There is no evidence of a simple factorial relation between them and the 
parent species. One of them, scintillans, must remain in the doubtful 
class until its chromosome numbers have been determined, but the 
inconstancy of this form suggests that it should be classed with lata 
and gigas under chromosome aberrations. The remaining five, Icevifolia, 
rubrinervis, nanella, oblonga and albida, are known to have 14 chromo- 
somes. Based on the evidence set forth in Chapter XII, it seems to 
us that one and only one category is open to these five forms and that 
probably albida, oblonga and most of the new forms that have appeared 
not only in cultures of lamarckiana and its derivatives but also in other 
species of Q^^nothera, are the result of chance recombinations of factors 
due to a condition of substrate hybridity. This expression, as has already 
been explained, is meant to imply that "mutating" species such as ffi". 
lamarckiana are merely species hybrids which happen to result from 
combinations of different reaction systems such that the majority of 



286 GENETICS IN RELATION TO AGRICULTURE 

their germ cells are similar. Hence they breed true in the main but 
occasionally throw the new combinations of diverse elements which have 
come to be known as ''mutants." 

In conclusion, it may be well to state our reasons for restricting the 
term, mutation, to those changes in specific factors, which result in the 
appearance of new Mendelizing characters. This term was used by de 
Vries to designate a more or less comprehensive change which appeared 
suddenly, without warning, giving the impression that a full-fledged new 
species had sprung from a pure, constant, old species much as Athena 
sprang from the head of Zeus. We cannot conceive of new species 
originating in this way except in certain exceedingly rare cases, which 
fall under the two categories already described and illustrated, viz., 
(1) single factor mutations having such a profound manifold effect that 
the new form would be generally recognized as a distinct species, and (2) 
chromosome aberrations during mitosis or meiosis. We have found that 
the majority of the new forms derived from (Enothera lamarckiana do 
not fall into either of these categories and that the most reasonable 
explanation of their origin is based on the assumption that (E. lamarck- 
iana is of hybrid origin. Therefore, if the term, mutation, is to retain 
the meaning originally given it by de Vries, we cannot continue to classify 
the majority of new (Enotheras or other organisms resulting from hybridi- 
zation as mutations. 

On the other hand, the fact that most discontinuous, inheritable varia- 
tions are caused by alterations in genetic factors and that these factor 
mutations play an important role as one means for organic evolution, 
seems to justify their recognition as mutations in the strict sense. By 
limiting the meaning of mutation as we propose all the objectionable im- 
plications previously connoted by the term are removed. The desir- 
ability of accomplishing this has been indicated by Agar, who states: 

"The greatest opposition to modern views of genetics has come from those 
who consider that they have taken away the philosophical basis of the theory 
of evolution and especially of the evolution of adaptation. For, while mutation 
could quickly bring about specific diversity, the evolution of complex adaptive 
structures is undoubtedly most easily grasped when the inheritable variations 
presented to natural selection are minute and abundant. This difficulty, though 
real, would undoubtedly have assumed smaller proportions had it not been for 
the natural fact that the earliest mutations studied were large morphological 
ones, and consequently that these have become fixed in many minds as types of 
mutational change." 

There is now abundant evidence that genetic diversity is expressed 
in minute morphological and physiological differences, and hence that 
mutations produce those small inheritable differences logically required 
for the explanation of adaptation through natural selection. 



PART II.— PLANT BREEDING 

CHAPTER XV 
HISTORICAL INTRODUCTION 

Plant improvement is nearly as old as agriculture. Our earliest agri- 
culturists must have protected the trees or plants that yielded food or 
shelter. Under protection the desirable forms among the chosen species 
were preserved. The finest example of this earliest plant improvement 
is found in rice, which has been cultivated for 5000 years or more in 
India and China and has long been grown in Egypt, East Africa, Japan, 
the Philippines, Java; Turkey and Italy. The remarkable plasticity of 
this species has enabled it to produce literally thousands of locally adapted 
forms. The oldest records of intentional preservation of superior plants 
are found, according to Darwin, in ancient Chinese encyclopedias that 
were translated by the Jesuits during the 18th century. The best plants 
and fruit trees were used for propagation; an imperial edict recommended 
the choice of large seed ; and even the Emperor Khang-hi is said to have 
originated the imperial rice by preserving and propagating a form which 
he noticed in a field. The original progenitors of our most important 
crop plants are mostly lost in antiquity, their descendants having been 
preserved by man's conscious or unconscious selection of desirable mu- 
tants or natural hybrids. 

The Beginning of Plant Breeding. — ^Long before any one thought of 
making a philosophical study of plant improvement the hybridization of 
flowers and the preservation of choice strains or favorite varieties was 
a common practice among gardeners and husbandmen. According to 
Fruwirth hybridization was practised in ancient times in China with 
various flowers, in Italy during the Roman Empire with roses, and in the 
17th century in Holland with tulips and primulas; and the artificial 
pollination of the female date palm was mentioned by Theophrastus 
as the beginning of the study of plant culture. The earliest syste- 
matic work in the production of new varieties, of which we have authentic 
records, was done by the Dutch flower fanciers. The hyacinth, ac- 
cording to Darwin, was introduced into England in 1596 and in 1629 
eight varieties were known. During the next hundred years or more 
the selection of varieties was carried on by the Dutch growers until, in 

287 



288 GENETICS IN RELATION TO AGRICULTURE 

1768 nearly 2000 sorts were known in Holland. But in 1864 only 700 
varieties were found in the largest garden in Haarlem, which fact in- 
dicates a gradual process of elimination of the less desirable selections 
of earlier years. 

Pioneers in Plant Breeding. — The systematic breeding of crop plants 
was begun in Europe during the latter part of the 18th century. Jean 
Baptiste Van Mons, a Belgian physician and professor of physics and 
chemistry in the University of Louvain, pursued plant breeding work as 
an avocation. But so great was his zeal in an effort to demonstrate 
certain theoretical ideas which he held concerning the improvement 
of fruits that the results of his labor were extensive. His experiments 
were begun in 1785. Thirty-eight years later he had 80,000 seedling 
trees in his "Nursery of Fidelity," as he called it, at Louvain. He dis- 
tributed cions without charge to many countries including America. 
He specialized on pears and his first catalogue, issued in 1823, lists 
1050 varieties. Altogether he originated nearly half that number. 
Van Mons' service to agriculture, especially to pomology, has been 
widely recognized.^ 

Three other pioneer breeders who began their work during Van 
Mons' life are Thaer, Knight, and Cooper, representing Germany, Eng- 
land, and the United States respectively. During the latter portion of 
the 18th and the earlier years of the 19th centuries each of these men 
carried on experiments in plant breeding and made contributions of 
tremendous importance to agriculture. Thomas Andrew Knight was the 
first to show the value of hybridization in plant improvement. Accord- 
ing to Bailey, in the variety, accuracy, significance, and candor of his 
experiments. Knight stands to the present day without a rival among 
horticulturists. He was also a successful breeder of livestock and author 
of papers on plant physiology and breeding. Albrecht Daniel Thaer 
also made hybridizing experiments but emphasized the value of selection. 
Plant breeding was only one of his many agricultural interests and he is 
credited with having laid the foundation of scientific agriculture of 
today. Joseph Cooper disproved the current fallacy as to the entire 
necessity for changing seed and showed the American farmer the impor- 
tance " of selecting seeds and roots for planting or sowing, from such vege- 
tables as come to the greatest perfection, in the soil which he cultivates." 
Like Van Mons, each of these men had his theories, but only experience 
revealed the truth in those theories. Based on their experience they 
formulated certain rules which they knew would yield results, but fre- 
quently the conclusions reached by them were only partially true. 

At least five other men deserve to be mentioned among the earlier 

^ For a discussion of Van Mons' theories and contributions (also of the work of 
Knight and Cooper) see Bailey, L. H.: "Survival of the Unlike," 1906, pp. 141-159. 



INTRODUCTION 289 

breeders of agricultural crop plants. Three of these worked with grains 
and two with fruits. John Le Couteur, during the early part of the last 
century, was raising what he supposed were pure and uniform varieties 
of wheat, when Professor La Gasca of the University of Madrid, after 
examining one of his fields, pointed out 23 distinct forms. This was the 
beginning of Le Coutcur's collection of 150 varieties of wheat, some of 
which were introduced to the trade. One of them, "Bellevue de Tala- 
vera," is still known as a pure and uniform variety. De Vries points 
out that Le Couteur simply assumed that the progeny of his selected 
plants would be like the parents and experience justified the assumption. 
Thus be became the first to discover the importance of selecting in- 
dividual plants in the improvement of cereals. 

Patrick Shirreff was also celebrated about the middle of the centurj^ 
as a breeder of cereals. His method differed from that of Le Couteur 
only in that he searched for very exceptional plants as the starting points 
of new varieties. During his lifetime he discovered seven new varieties, 
which according to Darwin, were grown extensively in Great Britain, 
but only four of them had permanent value. He also proceeded on the 
assumption that his single selected plants would breed true and each did 
so. According to de Vries, he considered the occasional appearance of a 
distinctly superior plant as merely accidental. 

Frederic F. Hallet, like Le Couteur and Shirreff, practised the rigid 
selection of individual plants in breeding wheat. Although he proceeded 
on the theory that by choosing the best spike on a certain plant and the 
best grain in the spike he would obtain corresponding improvement in 
the variety, yet he did not rely on mere apparent superiority, but tested 
each grain on each spike. He then selected the finest plant of all. He 
began his work in 1857 and made important introductions during the 60's. 
While Le Couteur and Shirreff assumed that the selection of a single 
plant was sufficient and thenceforth gave their attention to multiplying 
the new variety, Hallet practised continuous- selection within his selected 
strains. He obtained considerable increase in yield as a result of his 
early selections but little or no increase due to continuous selection within 
pure strains. The success of these three pioneer wheat breeders was 
unquestionably due to the fact that they practised the isolation of pure 
lines some of which were superior to ordinary varieties. 

Charles Mason Hovey was the "father of the American strawberry." 
As early as 1830 Hovey "had a list of 30 strawberries of his own origina- 
tion, all springing from the "Hovey," which, together with "Boston 
Pine," had been introduced a few years earlier. Hovey crossed a 
native American species with the imported "Pine" variety, which is 
supposed to have sprung from the beach or sand strawberry of the Pacific 
Coast some years after its introduction into Europe. Some of Hovey's 

19 



290 GENETICS IN RELATION TO AGRICULTURE 

new varieties stood the test of years and his work served to stimulate 
further efforts to improve the most important horticultural crop of 
America. 

Ephraim Wales Bull produced the Concord grape as a result of 
eleven years of patient work in crossing the native species, Vitis lahrusca, 
with European varieties, raising the seedlings and testing selections. 
"From over 22,000 seedlings there are 21 which I consider valuable," 
he writes. Although the hybrid nature of the Concord and other deriva- 
tives of Vitis lahrusca has been questioned, the evidence from extensive 
tests of selfed seedlings of this and several other standard American 
varieties as reported by Hedrick and Anthony seem to indicate that 
they are really hybrids between American species if not between V. 
lahrusca and V. vinifera. Whatever the origin of the Concord may 
have been, its sterling value is evidenced by its history. Introduced 
in 1853, "ten years later the Concord grape was spread over the entire 
northern part of the United States and is now widely used in the temperate 
regions of most parts of the earth." Ephraim Bull's service to his 
fellow men seems to have been all but forgotten while he was still living, 
since "he died neglected, in poverty, broken in spirit." Vast as would 
be the value of his contribution if it could be computed, even more 
valuable was the inspiration he gave, "which has helped to make plant 
breeding one of the great forces in cheaply feeding the world. ^" 

The demands and possibilities of developing agriculture aroused 
the ambitions of two far-sighted agriculturists — Martin Hope Sutton 
and Pierre Louis Frangois Leveque de Vilmorin. A student of botany 
from his boyhood, Sutton had already made improvements in a number 
of plants when the Irish potato famine of 1847 drew public attention 
to his work through the substitutes which he suggested for the devas- 
tated potato crop. Later on the introduction of the Golden Tankard 
mangel, the Magnum Bonum potato, and the Marrowfat pea helped to 
establish the high reputation which the firm of Sutton and Sons came 
to hold throughout the world. They greatly improved many flowers as 
well as crop plants. Sutton's "Permanent Pastures" is still a standard 
work on grasses. 

In 1843 Vilmorin took charge of the seed establishment which had 
already passed through the hands of two generations of this remarkable 
family. His father, Andre Leveque de Vilmorin, had conducted a selec- 
tion experiment with carrots about ten years earlier. Besides the main- 

1 The earliest hybridizers of grapes in America, according to Waugh, were Dr. 
Wm. Valk of Long Island (1845) and John Fisk Allen of Massachusetts (1846 or '47). 
Waugh also states that the two foremost American grape hybridists are E. S. Rogers 
of Massachusetts, who began in 1848 and distributed many numbered seedlings for 
trial in 1858, and T. V. Munson of Texas, who has probably added more to the prac- 
tical American fruit list in his hybrid grapes than has any other plant breeder. 



INTRODUCTION 291 

tenancc of work already iiiulor way, Vilmoriii undertook two projcctvS 
which yielded results of the greatest importance to the entire worhl. 
One was the collection of wheats and other grains from many countries 
in order to compare them and to select those of greatest value. In con- 
nection with this work on grains he invented the pure-line method of 
selection and progeny test which came to be known as the "Vilmorin 
Method" and which has been used so successfully with wheat and other 
self-fertilized plants. From our present knowledge of pure lines we 
can understand why this method was effective. Vilmorin's other im- 
portant project was the improvement of the sugar beet. Previous to 
1850 the beets had been selected according to form only. This method 
of selection began as early as 1787 on the seed farm of H. Mette in Qued- 
linburg, Germany, according to Legier. Selection on the basis of specific 
gravity was practised from 1850 to 1862, when the method of determining 
sugar content by means of polarized light was introduced. His success 
made beet-sugar production a commercial i)()ssibility and gave his name 
an enduring place in history. 

More Recent Progress in Plant Breeding. — The great world move- 
ments of the 19th century following the improvement of transportation 
facilities, the migration of peoples, industrial development and the growth 
of international trade, together with the improvement of farm machinery, 
resulted in the extension of agricultural industries and gave a greater 
impetus to plant breeding. This activity was manifested first in Europe 
and later, particularly in the United States Department of Agriculture 
and the state experiment stations, in America. Naturally the efforts 
at improvement were concentrated in the main on the crop plants pro- 
ducing the raw materials of importance in the world's markets, such 
as wheat and other small grains, sugar beets, corn, cotton, forage plants, 
the apple and other fruits. The methods employed were those which 
had been used in the past for the most part, but they were systematized 
and combined for more effective utilization. These methods may be 
classified under the following heads: 

1. Mass selection. 

2. Line selection antl progeny test. 

3. Hybridization followed by direct utilization or selection and 
fixation of new varieties. 

4. Clonal selection. 

Mass Selection. — The method of mass selection consists simply in 
picking out choice plants from the main crop and sowing the seed from 
them 671 masse. It has long been used, especially in improving small 
grains, but it has also been used with many other crops. With this 
method it has usually been found necessary continually to repeat the 



292 GENETICS IN RELATION TO AGRICULTURE 

selection of best plants in order to maintain the improvement already 
gained. One of the earliest breeders to use this method was Andre 
Leveque de Vilmorin, who began selecting carrots about 1830. Soon 
thereafter selection of sugar beets for seed production was begun in 
France and Germany, first according to form of the root alone, but later 
according to specific gravity and actual analyses of sugar content. 
Mass selection later became the principal method of improving small 
grains in Germany, and it has been known as the German method of 
"broad breeding." The earliest prominent breeder of small grains was 
W. Rimpau, who began his work with rye in 1867 and developed the 
famous Schlanstedt variety. Later he worked with wheat extensively, 
first by mass selection and, more recently, by hybridization of varieties 
and subspecies. Although there have been scores of successful breeders 
of each of the important small grains in Germany, Rimpau was the 
first to engage in this work on a large scale. 

Mass selection in maize was begun as early as 1825, when J. L. 
Leaming, of Ohio, began the selection of best ears from his field for seed 
corn. By repeating this process he soon developed a superior strain 
that came to be known as the Leaming variety. The same simple 
method was employed in originating Ried Yellow Dent (1847), Morley 
Prolific (1876), and Boone County White (1885). The famous Illinois 
corn-breeding experiments, which will be described in later chapters, 
were begun in 1896 by Cyril G. Hopkins, then Professor of Agronomy 
in the University of Illinois. Among the other investigators who have 
participated in this undertaking are East, Shamel and L. H. Smith. 
The general result of the project has been the most convincing proof of 
the efficacy and practicability of mass selection in corn, not only for the 
chemical and physical characters of the grains but for other characters 
of the corn plant as well. 

The improvement of cotton by mass selection has doubtless been 
practised for centuries. Authentic records of the earlier methods used 
in foreign countries are scarce, but the characteristic variability in 
length of fiber, combined with the very practical value of increasing the 
average length, must have appealed to growers, at least in the more prog- 
ressive cotton growing regions of the world. In the South Carolina islands 
according to Webber the sea island types of cotton have been developed 
by consistent mass selection for early maturity, increased length of lint, 
and greater productiveness from a West Indian perennial type which 
was originally unsuited to conditions under which its derivatives 
•are now grown so successfully. Mass selection in cotton has been 
resorted to also in the campaign against various plant diseases, 
particularly cotton wilt, and for early maturity to avoid the ravages of 
the boll weevil. 



INTRODUCTION 293 

Line Selection and Progeny Test. — Turning now to the .second of the 
four general methods, we find that the progeny test of individual plants 
was first used by Le Contour and Shirreff . But it was Louis de Vilmorin 
who first gave special attention to the value of the progeny test (1856) 
and, contemporaneously with Hallet, practised the selection of single 
plants, i.e., of pure lines in wheat, oats and barley, followed by separate 
tests of their progeny. This method was first used in America by Willet 
M. Hays who began the improvement of small grains at the Minnesota 
Experiment Station in 1888. Convinced by the results of extensive 
variety tests that systematic breeding would be required in order to 
secure a marked increase in yield of first class wheat. Hays devised the 
centgener method of grain breeding, which, briefly, consists of planting 
about 100 seeds from each selected plant in trial plots; the more promising 
centgeners being selected for testing on a larger scale. Hays' work re- 
sulted in the isolation in 1892 of two plants whose progeny within a 
decade were grown on thousands of acres. Although many new strains 
were secured, the rigid tests of several consecutive years in which the 
most promising strains were compared with each other and with the best 
commercial varieties, resulted in securing but few really superior varieties. 
However, these made possible an increased production of wheat through- 
out the northern states and in Canada. 

The Swedish Seed Association was organized in 1886 and established 
an experiment station at Svalof. During the first 5 or 6 years only 
mass selection was practised, but soon after Hjalmar Nilsson became 
director in 1891 the ''Vilmorin Method" was introduced. At Svalof 
it came to be known as the "System of Pedigree" or ''Separate Culture." 
Nilsson was led to adopt this system as the method for originating 
new varieties by the accidental discovery that the only wheat plots 
that were entirely uniform were grown from single plant selections. The 
new varieties produced at Svalof are now grown throughout the agricul- 
tural portion of Sweden. This station is also engaged in the systematic 
improvement of peas, clovers, grasses and potatoes. All this work is 
based on mass and line selection followed by field tests and distribution. 

The first application of the pure-line conception to a naturally cross- 
fertilized plant was made by Shull and by East working independently 
with corn. By guarding and self-pollinating individual plants for suc- 
cessive generations, a number of morphologically distinct strains were 
isolated, thus proving that the original population was a mixture of 
biotypes. These same methods, however, had been employed for a 
number of years by Webber, Hartley, and probably others in working 
with corn, cotton, and other naturally cross-fertilized plants. In recent 
years the plant-row test has been used for the improvement of old 
strains or production of new ones. In Germany, von Lochow in 1894 



294 GENETICS IN RELATION TO AGRICULTURE 

adopted a modified form of line selection in the improvement of rye, 
which is also naturally cross-fertilized. 

Timothy breeding was undertaken by the New York (Cornell) Experi- 
ment Station, under the direction of Hunt, Gilmore and others in 1903. 
To begin with, samples of seed were secured from 22 states and 11 foreign 
countries. Although it had been long cultivated in certain parts of 
Europe, there were no distinct varieties of this species of grass {Phleum 
pratense) because it is normally cross-fertilized. Many interesting 
variations were found among the plants grown from the various samples, 
some of them being of great commercial value. After several years of 
experimental work 17 new sorts were selected as most promising. These 
had been increased vegetatively by division and subjected to progeny 
tests with both cross-pollinated and self-pollinated seed. In two years 
tests the 17 selections gave an average increased yield of 36% per cent, 
above ordinary timothy. If such an increase in production of timothy 
could be extended throughout the country, it would, according to Webber, 
add over .S90,000,000 to the value of the annual hay crop. 

Hybridization. — The third general method of plant breeding in the 
light of genetical science holds great promise of future possibilities. 
In spite of Knight's early demonstration of the value of varietal crosses in 
breeding, this method did not come into general use until the latter part 
of the nineteenth century. According to Darbishire, another English 
horticulturist, John Goss, made some of the identical crosses used by 
Mendel, and noted the phenomena of dominance in Fi and recombination 
in F2, but failed to grasp the significance of the facts he observed. Accord- 
ing to Munson, it was the horticulturist, A. J. Downing, who in 1836 
first called the attention of American breeders to the possibilities in 
hybridization. After his success with strawberries, Hovey, in 1844, 
definitely championed the cause. The achievements of Hovey, Downing 
and others soon led to the general adoption of cross-fertilization as a 
method of breeding. In their efforts to secure varieties having certain 
combinations of desirable characters, the crossing of varieties of small 
grains was employed to advantage by Rimpan, Blount, Pringle, Hays, 
Nilsson and others in later years. The remarkable Marquis wheat 
which has proved so valuable in the northern wheat regions is a hybrid 
according to Carleton which was probably made by A. P. Saunders at 
the Agassiz (British Columbia) Experiment Farm in 1892. The applica- 
tion of this method in the production of disease resistant commercial 
strains has been attempted. R. H. Biffin began his study of wheat breed- 
ing in 1909 in the service of the National Association of British and Irish 
Millers. The demand was for a beardless, rust-resistant variety of high 
yielding power and good milling quality. Not being able to discover any 
single variety which combined all these characters. Biffin attacked the 
problem from the Mendelian standpoint and has attempted to secure the 



INTRODUCTION 295 

desired combinations through the liyl)ri(Hzation of a low (luality, rust-re- 
sistant form with a variety very susceptible to rust but whose characters 
are otherwise superior. Biffin found that susceptibility to yellow rust 
(Puccinia glumarum) is dominant to rust resistance, in the cross between 
Rivet and Red King, but that resistant forms appeared in the F^ genera- 
tion which bred true for resistance. This discovery marks a definite 
forward step in the breeding of disease resistant plants. However, the 
problems of disease resistance are complicated by the variability of the 
parasitic organisms involved. 

Hybridization of maize was begun as early as 1878 at the Michigan 
experiment station and was taken up from time to time at certain other 
stations. In 1900 the U. S. Department of Agriculture began a large 
series of experiments in crossing corn, using "all types obtainable." 
This work has resulted in the distribution for trial of many promising 
selections. Following the striking experiments of East and Shull in 
crossing strains of corn that had been inbred for several generations, 
many experiment stations began the crossing of varieties and strains for 
increased production as well as for new combinations of characters. 

With cotton, the recent work of Balls in Egypt has furnished a basis 
for the pedigree and hybridization method of breeding. Although cotton 
is self-fertilized to a large degree, yet it is visited by insects during the 
early morning hours so that there is always a certain amount of natural 
crossing. It is very susceptible to environmental effects and its chro- 
mosome number is large (haploid number 20). These conditions make 
improvement by crossing a difficult matter. Cook noted the fact 
that parent characters are sometimes intensified in the Fi in cotton and 
recommended the use of Fi hybrid seed of proved crosses as a means 
of enhancing the quality of the lint. He also suggested a practic- 
able and economical method of producing and utilizing such hybrid seed. 

Apple breeding by crossing varieties was begun by Knight but this 
method has not been used extensively in Europe. In America the cross- 
breeding of apple varieties probably was begun by Charles Arnold of 
Ontario, Canada, about the middle of the last century. Other early 
hybridizers who worked with varieties of the common apple, Pyrus 
malus, were F. P. Sharp of New Brunswick, who began crossbreeding 
in 1869 and C. G. Patten of Iowa, who commenced somewhat later, but 
who has worked continuously with apples and pears since 1879. In 
this connection recognition is due Peter M. Gideon and the host he 
represents, who have produced new varieties of apples b}' raising seedlings 
and selecting the best. Most of the new sorts obtained in this way are 
of hybrid parentage. More recently important work on variety crossing 
of apples has been done by Macoun in Canada, Hedrick in New York, 
and Evans in Missouri, 



29G GENETICS IN RELATION TO AGRICULTURE 

The composite crossing of three or more varieties in an attempt to 
effect a desired combination has been used successfully in small grains, 
as well as in many flowers. Referring to grains alone, William Farrer 
of Australia, A. N. Jones of the United States, and John Garton of 
England have used this method successfully. In the opinion of Carleton, 
Farrer leads all breeders in the production of hybrids that have come 
into practical use. He continually practised composite crossing, as 
many as six different varieties or subspecies entering into the ancestry 
of many of his new sorts, some of which are of superior production or 
milling quality as well as being disease resistant. 

Interspecific hybrids have frequently been produced by breeders 
seeking some definite goal, occasionally with striking success, especially 
among fruits. Even intergeneric hybrids have been reported, but the 
known cases, with the exception of orchids, are of slight importance to 
agriculture. For example, van der Stok secured a fertile hybrid between 
corn and teosinte in the hope that some of the hybrids would bear good 
sized ears and be resistant to chlorosis, a hope which was not, however, 
realized. A few similar cases are known, particularly among cereals, 
but very little use has been founcF for them. In fact, utilization of wide 
crosses is rather definitely restricted to direct employment of the Fi 
in cases where conditions of seed production are particularly favorable 
for producing large quantities of hybrid seed or where the hybrid may be 
propagated by clonal multiplication. 

Alfalfa culture appears to be capable of still further extension through 
crossing of species. According to Fruwirth hybrids between common 
alfalfa, Medicago saliva, and M. falcata are easily produced and occur 
abundantly wherever plants of the two species grow near each other, 
the crossing being effected by insects, especially bees. This was reported 
in 1877 by Urban. These hybrid forms are known as M. 7nedia Pers. 
{M. varia Martyn., M. versicolor Ser.). Seeds of these hybrids produce 
forms that can be considered M. 7nedia, and, while flower color and pod 
form are inconstant, the plants bear more seed and grow more luxuriantly 
than M. falcata and adapt themselves to varied soil conditions. West- 
gate has found good evidence that the well known hardy Grimm alfalfa 
originated as a natural hybrid between these species, and that it was not 
a product of acclimatization. Piper in 1908 called attention to the desir- 
ability of securing hybrids between M. sativa and the yellow-flowered 
Siberian species, and Hansen has recently determined the practicability 
of producing such hybrids on a large scale by mixed field plantings. 
Experts of the Bureau of Plant Industry of the U. S. Department of 
Agriculture have made an enormous number of attempts to cross different 
species of Medicago but utterly without success except in the case of 
falcata and satim. Selfed mtiva and especially media, {falcata x sativa) 



INTRODUCTION 297 

give many aberrant forms. A very common one under greenhouse 
conditions is a form with very short internodes and very small leaves. 
This is presumed to be the form which Southworth mistakenly reported 
as a hybrid between M. sativa and M. Inpulina. 

In various fruits and in many flowers the crossing of species has 
yielded many valuable varieties. Some cases among flow(M's will be 
discussed in the following chapter. Among tree fruits the next hybridi- 
zers of species after Bull were the men who undertook to combine the 
hard}^ character of the Russian apples, which had been introduced during 
the 80's. Dr. William Saunders, then Director of the Dominion Experi- 
mental Farms, began this work in 1894. Similar work, with apples, 
cherries, plums, etc., has been carried on very extensively, and already 
with important results, by N. E. Hansen of the South Dakota Experi- 
ment Station. The production of a list of peach varieties adapted to 
the Gulf Coast States was the work of H. H. Hume, then of the Florida 
Experiment Station, and of P. J. Berckmans in Georgia. This was 
accomplished largely through the hybridization of the Chinese Saucer 
or Peen-to peach, Amygdalus platycarpa, with commercial varieties of 
the common peach, Amygdalus persica. The work of Webber and 
Swingle with crosses between various species of Citrus has received inter- 
national recognition, not only because of the results secured but on 
account of the possibilities in the improvement of citrous fruits which it 
revealed. The production of aphis-resistant plums among hybrids of 
distinct species, as reported by Beach and Maney, exemplifies an impor- 
tant line of attack in breeding disease-resistant plants. 

No small part of Luther Burbank's fame is due to his success in 
crossing species. Among the many interspecific hybrids which he pro- 
duced should be mentioned plumcots (hybrids between plums and apri- 
cots), the Royal walnut {Juglans Calif ornica X J. nigra), the Primus 
and Phenomenal berries (hybrids between species of Rubus) , many valu- 
able plums and a host of flowering plants. In his work with plums, 
as well as in the production of certain flower novelties, Burbank practised 
composite hybridization. An illustration taken from de Vries' account 
of Burbank's work, is the pedigree of the Alhambra plum, shown in Fig. 
119. 

An equally if not more important phase of Burbank's work is his 
discovery of novelties and his perfection of the same by means of selection. 
His method is hardly to be classified as mass selection, nor is it line selec- 
tion in the strict sense. An important feature has been the use of very 
large numbers of seedlings either of introduced species, commercial 
varieties, or his own hybrids. It is by the use of his unusual power of 
observation, which Wickson thinks amounts to a gift of intuition, in 
choosing say a dozen seedlings from as many thousand, that this one man 



298 GENETICS IN RELATION TO AGRICULTURE 

has accomplished so much. His methods of hybridization, also, have 
been such as to economize time rather than insure certainty as to ancestry 



d. 



Alhambra 



Nigra 
[ Americana 
[c < 



Triflora 

Simoni 

French Prune 

[ Pissardi 

[a I 

Kelsey 



Fig. 119. — Ancestry of the Alhambra plum. 

His aim has always been the tangible result rather than advancement of 
scientific knowledge. 

Clonal Selection. — Under the term clonal selection is included all 
methods of plant improvement based upon the utilization of asexual 
means of multiplication, whether by selecting the most favorable clones 
from a mixed population, or by selecting and propagating favorable varia- 
tions within clones. In potatoes many commercial varieties are definitely 
known to be mixtures of different clones, and improvement may be 
effected by simply selecting those which are most productive and most 
desirable from a market standpoint. A unique instance of clonal 
selection is that followed in Oklahoma and other regions along the north- 
ern limits of the range of Bermuda grass. There the cold winters kill 
off the less hardy strains ; those that remain are propagated by distribu- 
ting sod. In alfalfa many improved strains have been produced by 
the selection and multiplication of superior individuals. This work has 
been carried on by the Bureau of Plant Industry of the U. S. Department 
of Agriculture and various stations, especially those in South Dakota, 
Kansas and Arizona. The propagation of improved strains by means 
of cuttings is of great practical value, and Hansen recommends the use 
of tobacco planting machines for the setting of rooted alfalfa cuttings. 

A phase of clonal selection which has recently come into prominence 
is hud selection, although the occurrence of bud variations, particularly 
of bud sports, has long been a matter of common knowledge. Munson 
(1906) seems to have been the first to call attention definitely to the pos- 
sibilities in fruit improvement by selection of buds from superior indi- 
viduals or vegetative parts, although Bailey had on several occasions 
previously pointed out that varieties sometimes originated from buds. 
During the past ten years many practical experiments in bud selection 
have been conducted, but with diverse results. 



INTRODUCTION 299 

The aim of the foregoing review has been to present the more promi- 
nent historical examples of the four general methods of plant breeding. 
Further details can be obtained from the authors cited and from Fru- 
vvirth's Die Ziichtung der landwirtschajtlichen Kultiirpflanzen (The Breed- 
ing of Agricultural Crop Plants). This useful work, consisting of five 
volumes, is partly in its second and third revised editions and is the most 
complete and thorough treatise on plant-breeding methods. 

Organization of Plant-breeding Work. — Growing appreciation of the 
importance of plant improvement to agriculture has led to organized 
effort along certain lines, some of which are discussed briefly below. 

Seed and Plant Introduction. — The first teacher of plant breeding in 
America was also her first agricultural explorer. In 1882 Budd went to 
Europe and Asiatic Russia for the purpose of studying horticultural 
problems. He was accompanied by Thero Gibbs of Canada, and the 
expedition was financed by the Iowa State Legislature and the Canadian 
Government. As a result of this exploration many hardy shrubs and 
trees were introduced into America. The Russian cherries and apples 
were of especial importance as they have been used, notably by Saunders 
and Hansen, in the production of new varieties, which are sufficiently 
hardy to resist the cold winters of the northwest portion of the great 
interior plain, Bailey, in 1894, called attention to the similarity in 
climates and floras of eastern America and eastern Asia and emphasized 
the "abundant reason for looking toward oriental Asia for further 
acquisitions, either in other species or in novel varieties." His wise 
foresight in this matter has received repeated verification in the numerous 
valuable introductions of Wilson and of Meyer. About this time the U.S. 
Department of Agriculture began to give serious attention to the intro- 
duction of seeds and plants from foreign countries under the supervision 
of Galloway. A few years later this important work was put in charge 
of Fairchild who has organized the present efficient system of agricultural 
exploration, seed and plant introduction, trial gardens and distribution 
of promising material. 

Collections of Plant-breeding Material. — The importance of bringing 
together a working collection of all available species and varieties within 
a group in which improvement is desired has been increasingly appreci- 
ated since the work of Vilmorin. The importance of local variety trials 
has long been realized and the collections of cultivated varieties at various 
experiment stations have proven very useful for purposes of selection of 
better adapted sorts as well as for some work in hybridization. Well- 
known examples are the sweet pea, peony, and chrysanthemum collec- 
tions at Cornell University and the collections of apples, plums, and 
grapes at the Geneva, N. Y., Experiment Station. But, on account of the 
time and expense involved in the work of hybridization, it is highly im- 



300 GENETICS IN RELATION TO AGRICULTURE 

portant that the most promising forms which exist be secured, if possible, 
at the beginning of such projects. Some of the older collections of living 
plants, such as Arnold Arboretum and the New York, Brooklyn, and 
Missouri botanical gardens, as well as the Government Office of Seed and 
Plant Introduction, have given valuable assistance in supplying new and 
rare material to breeders. The transportation of pollen has also been 
resorted to, especially by the U. S. Department of Agriculture in its 
breeding of Citrus and it is known that, with proper precautions, some 
kinds of pollen can be sent by mail half-way around the world and still 
be viable. However, certain much desired crosses can be secured only 
after repeated efforts and the trial of various methods. Moreover, the 
response of introduced forms to local conditions is a most important con- 
sideration. All too often a supposedly promising new plant has proven 
entirely unfit for certain localities. These considerations are leading to 
the establishment of large working collections of our more important 
semi-permanent crop plants, especially the tree fruits. For example, the 
University of California Citrus Experiment Station is accumulating a 
collection which will include all the known species and varieties of Citrus 
and allied genera which will endure local conditions. 

Research on Plant Groups. — A breeding program such as that con- 
templated by the institution just mentioned involves the necessity of ex- 
tensive botanical investigations. In this particular instance it is fortu- 
nate that extensive work has already been accomplished by the U. S. 
Department of Agriculture since already a large amount of data on the 
botanical relationships and geographical distribution of the members 
of the Citrus group has been collected. As a result of these studies and 
explorations several new and very promising forms have been introduced 
and have already been utilized in breeding experiments by the Bureau of 
Plant Industry. Similar investigations of the genus Prunus are also 
under way by the Department. The recent explorations of date growing 
countries and studies on the varieties of dates is another illustration of 
the sort of work that is needed, not only among fruits in general but in the 
field crops as well. 

Organization of Plant Breeders. — In December, 1903, the American 
Breeders Association was organized under the auspices of the American 
Association of Agricultural Colleges and Experiment Stations. During 
the first seven years of its existence the publications of this organization 
were restricted to the annual reports of its meetings. These reports 
contain the papers which were presented at the meetings either in full or 
by title. In 1910 the Association undertook the publication of a quar- 
terly journal, the American Breeders Magazine, and discontinued the 
publication of annual reports. This magazine in January, 1914, became 
the Journal of Heredity, which is published monthly. At the same time 



INTRODUCTION 301 

the American Breeders Association changed its name to the American 
Genetic Association. With its policy of unrestricted membership from 
the beginning this organization has done great service in fostering the 
common interests of geneticists and practical breeders. There are 
state associations of plant breeders in New York, Wisconsin, Minnesota, 
Illinois, Pennsylvania, Ohio, Nebraska, and Georgia. In certain other 
states the agricultural and horticultural societies have fostered plant- 
breeding work to a greater or less extent. The meetings held with their 
addresses and discussions, the exhibits of new introductions and occasional 
tiemonstrations in plant improvement by the experiment station or other 
agencies, have aided in bringing to the seed growers and farmers of the 
United States the knowledge of superior plants and their practical value. 
The Canadian Seed Growers' Association has fulfilled a similar mission. 
Summary. — Starting with the sporadic efforts of a century or more 
ago to find some better varieties of fruits and grains, there has been a 
gradual broadening of the great movement to increase agricultural 
output and raise the quality of raw materials by means of plant improve- 
ment. Throughout the later stages of this development scientific 
knowledge has become increasingly important until now the specialist 
on a particular crop plant may invoke the aid of every branch of agri- 
cultural science in selecting his material for breeding operations, making 
the desired crosses and selecting the progeny. All this has been done 
without much, if any, definite knowledge concerning the heredity of the 
plant in question. Within a decade the science of genetics has developed 
to a stage where it is capable not only of furnishing a rational explana- 
tion for the phenomena of variation and heredity which in the past 
seemed obscure and contradictory, but also of guiding the breeder 
who will familiarize himself with the established principles of the science, 
so that he may reach his goal with greater speed and economy. It is 
the purpose of the following chapters to set forth these principles in 
as clear and practical a manner as possible. It will be assumed, of 
course, that the reader is familiar with the fundamental treatment of 
the preceding chapters. 



CHAPTER XVI 
ON VARIETIES IN PLANTS 

The multiplicity and diversity of the varieties of cultivated plants never 
fail to impress the thoughtful observer. The cereals, fiber plants, legumes, 
root crops, and tree fruits which comprise most of the important agricul- 
tural crop plants include some 30 species. It is safe to assume that within 
this small group of species over 5000 distinct varieties are known at pres- 
ent. Of rice alone there are thousands of varieties in cultivation. Among 
flowering plants we find the same diversity. The rose, lily, chrysanthe- 
mum, violet, carnation, sweet pea, dahlia, gladiolus, tulip, and hyacinth 
of our gardens and greenhouses represent not more than 200 species, 
while of roses alone as many as 1000 named varieties are now listed in 
European catalogues. In general the longer and more widely culti- 
vated species contain the larger groups of varieties, partly because of 
the greater opportunity for their discover}^ and partly because these 
species have been subjected to conditions most favorable for the pro- 
duction of varieties. Before attempting to discuss the conditions or 
operations that lead to the production of new varieties it is necessary to 
enquire into the natural processes by which varieties have been produced. 

The Origin of Domestic Varieties of Plants. — Agriculturists have 
made use of three general methods in creating new varieties of cultivated 
plants, viz.: (1) the utilization of mutations or sports; (2) the employ- 
ment of hybridization and selection methods; and (3) the utilization of 
clonal diversity. The utilization of mutations should be interpreted 
to include not only the discovery and multiplication of mutant forms, 
but also the recombination of mutant characters in new varieties by 
hybridization. We include selection in the same category with hybrid- 
ization, because according to the hypothesis which we have championed 
throughout this text, its effectiveness usually depends upon the existence 
of germinal diversity such as follows hybridization. In certain cases, 
of course, selection methods have depended for success upon the utiliza- 
tion of mutations having minor character effects. The origin of varieties 
by these three different methods may be illustrated by considering in 
some detail the horticultural history of certain plants. Since the 
ancestors of most of our crop plants are now extinct, we may turn for 
this purpose to some of the more recently domesticated species, the 
histories of which are known more precisely. 

302 



ON VARIETIES IN PLANTS 



303 



Origin of Sweet Pea Varieties. — The sweet pea, Lathyrus odoratus , 
provides an excellent illustration of the origin of varieties by the utili- 
zation of mutations. Its history as a horticultural plant is known from 
the beginning and has been thoroughly reviewed in publications of the 
Cornell Station. The sweet pea was introduced into Holland and 
England from Sicily via Italy in 1699, and was first illustrated in a 
description published in 1700. The drawing is reproduced in Fig. 120. 
It will be noted that in habit it was similar to the cultivated sweet peas 
of the present day and the height to which it would climb was "6 or 




Fig. 120. — -Commeliii's drawing of the sweet pea in Hort-Medici Amstelodamensis, 1700. 

(After Bcal.) 



7 feet," but the flower stems were short and bore only two flowers, while 
the flowers themselves were relatively small, with erect or reflexed 
standard and conspicuous, depressed wings. In color the standard 
was reddish purple and the wings light bluish purple. From this modest 
beginning there have been developed several distinct types of plant and 
flower forms and a list of named varieties, even within the most highly 
developed type of flower (the Spencer or waved form), which includes 
over 500 entirely distinct colors, tints, shades, and combinations. 
By far the greatest amount of this work has been accomplishetl during 
the past 50 years, during which period hybridization has been used 



304 GENETICS IN RELATION TO AGRICULTURE 

extensively in creating improved varieties. But before hybridization 
was resorted to there were a dozen distinct color varieties which had 
arisen by mutation. Besides color mutations there have occurred 
spontaneous changes in flower form, flower size, and number of flowers 
on the stem, in stature and habit of the plant and in season of bloom, 
some of which are described below. 

Flower Color in Sweet Peas. — The chronology and probable ancestry 
of the color varieties of the sweet pea which appeared during the first 
180 years of its horticultural history are shown in condensed form in 
Table XLV. This summary is based upon Beal's excellent historical 
review, from which citations to original sources have been obtained. 

Apparently the course of events was about as follows. From the 
original type form there appeared first white mutations (Plate III, 3.) 
If we call the simple flower-color factor complex CRB, in which C and R 
are complementary factors producing red, and B an epistatic factor 
which modifies that color to purple, then these mutations apparently 
depended upon a change in either C or Rto the recessive, white condition. 
The Painted Lady variety, red instead of purple, shown in Plate III, 2, 
appeared very soon after this, apparently as an independent mutation 
in the factor B from purple. By the close of the eighteenth century 
two other color types, black and scarlet, had been added to the list. 
The wild form and Painted Lady are bicolors, that is, the wings are lighter 
in color than the standard. The new color type scarlet (Plate III, 5,) 
apparently resulted from a recessive factor mutation conditioning the 
development of full color in the wings along with a certain intensification 
of color in the standard. Black (Plate III, 6,) was probably also merely a 
factor mutation for more intense pigmentation from the wild color type. 
Early in the eighteenth century a "blue" form, var. caenileus, was 
described in the trade, but its genetic relationships have not been clearly 
defined. Plate III, 8, which is taken to represent it has not been copied from 
a particular variety as was done in the case of the other types. Further 
additions shortly followed in the form of a "striped" variety, and of a 
"yellow" variety. The latter (Plate III, 4) unquestionably originated as 
a factor mutation from white, the former may have arisen as a factor 
mutation in purple. Plants with primrose yellow flowers have since been 
observed a number of times in white cultures, but never in red ones. 
This practically closes the account of the origin of color mutations up to 
the year 1880, after which time hyl^ridization was resorted to extensively 
in the creation of new varieties. 

Form and Size in Sweet Peas. — The changes in form and size of flower 
in the sweet pea have been no less striking than those in color, and they 
have been responsible for a large portion of the popularity which it 
enjoys. Today one can scarcely recognize in the favorite varieties of 




Plate III. — Oldest Varieties of the Sweet Pea. 

1. The original wild form. 2. Old Painted Lady. 3. White. 4. Yellow. 5. New Painted Lady or 
"Scarlet." 6. "Black." 7. "Blue Edged" ( = purple picotee?"). 8. "Blue." 

Numbers 4-8 >ie reconstructions based on modern varieties because the original varieties listed 
under these names cannot be identified with absolute certainty. But it is highly pi obablc that they 
were very similar to the types shown above and that they originated by mutation in the order 
indicated. 



ON VARIETIES IN PLANTS 



30'i 



Table XLV. — Okigin of tue Earlier Color Varieties of the Sweet Pea^ 



1700 

1718 white- 

(cRBE or CrBE) 
1 73 1 white 



purple and blue — CRBE 



1793 



180G 



1817 



white 



white 





1824 yellow white 

• (= primrose?) 
1840 white 



1845-49 



1850 



1860 yellow 

( = primrose?) 
1865 



Painted Lady 
( = pink and 
white) CRbE 



Painted Lady scarlet- 

I ( = deep rose) ( = dark violet) 

Old Painted New Painted dark purple 
Lady Lady or 

Scarlet 



Old Painted 
Lady 



New Painted 
Lady 



Scarlet In- 
vincible 



dark purple 

and deep 

violet 

New Large 
Purple 

New Large 
Dark Purple 



striped 

(= purple 

with brown, 

lavender or 

white?) 



New Striped 



blue 



dark bluish 
purple and 
pale blue 



Blue Edged 



Blue Edged 



1870 
List 



f yellow, white Painted Lady, Scarlet In- black, Imper- purple 

{Crown Prin- vincible, ial Purple, striped 

cess of Prussia scarlet, scar- black with with white, 

(pink and rose let striped light blue, 

pink). with white. 

yellow, white Painted Lady, New Painted Black, Black purple striped Blue Edged^ 



1880 
List 



Crown Prin- 


Lady, scarlet, 


Invincible, 


with white, 


Butterfly^ 


cess of Prus- 


Scarlet Invin- 


black with 


Invincible 


Captain 


sia, Fairy 


cible, scarlet 


light blue. 


Striped Violet 


Clarke* 


Queen ( = 


striped with 


Large dark 


Queen. 


Hetero- 


pink on 


white. 


purple. Im- 




sperma^ 


white). Queen 




perial purple, 






( = light pink 




Purple In- 






and pink pur- 




vincible. 






ple). 











1 In each case the color of the standard or banner is given first and of the wings second; the descrip- 
tive terms and variety names are identical with those in the original descriptions. 

- Described by Bailey and Wyman as purple-lilac in color ( = purple picotee). 

' Quite similar to Blue Edged according to Beal ( = purple picotee). 

4 = "white merging into pink and purple, wings white with purplish cast, wings edged with blue" 
( = purple picotee) . 

6 No description available; mottled seeds? 
20 



306 



GENETICS IN RELATION TO AGRICULTURE 



the garden traces of the early pecuhar form of the flower portrayed in 
Plate III. In the original form the standard was erect, narrow at the 
base, notched at the top, and reflexed or slightly rolled at the sides. 
From it have been derived three distinct flower types; the grandiflora, 
the hooded, and the popular waved Spencer forms. The origin of the 
first two named is in some doubt. The hooded character was found in 
some of the earlier varieties. It was sometimes associated with notches 
in the sides as in the Butterfly (Fig. 121), and this character is found also 




Fig. 121. — Forms of sweet pea flowers — the standard or banner. Open or grandiflora 
form (upper row left to right) — Alba Magnifica, Shasta, Golden Rose. Hooded form (middle 
row) — Butterfly, Admiration, Dorothy Eckford. Waved form (lower row) — Elsie Herbert, 
Apple Blossom Spencer, White Spencer. (From Beal.) 

in some of the present day favorites. Bateson reports that hooded is 
recessive to grandiflora or erect type of standard. Some of the earliest 
varieties of improved grandiflora form were Queen of England (1888), 
Blanche Ferry (1889) and Alba Magnifica (1891). The waved or Spencer 
form is of more recent origin, and authorities are agreed that it arose as 
a "sport" from a beautiful, pink, hooded variety, Prima Donna. The 
pronounced waviness of standard and wings which characterizes this 
type had not appeared before in sweet peas. 

The two upper series in Fig. 121 indicate the more recent progress in 
enlarging flower size. Alba Magnifica and Butterfly were great acqui- 



ON VARIETIES IN PLANTS 



307 



sitions in their day iiiid were doubtless considerably larger than the 
oldest varieties. The first definite reference to size is found in New 
Large Purple, listed in 1845. As this occurs in the darkest color group 
and 15 years before the hybrid origin of a new variety, ]Mue Edged, was 
even suggested, it probably represents a factor mutation. That such 
nuitations ac^tually occurred in the sweet pea is proved by the fact that 
Countess Spencer and (iladys Unwin were both decidedly larger than 
Prima Donna fiorn the very first. The same is true as regards number 
of flowers in the cluster. Prima Donna, according to Beal's description, 
bore two or three, usually three, flowers on a stalk, while Countess 
Spencer has three to four flowers in a cluster. Many of the recent Spencer 




Fig. 122. — On the left, Suapdnigon sweet peas. On the riglit, double sweet pea, White 

Wonder. {From Beal.) 

varieties bear almost uniformly four-flowered clusters. The original 
form and earliest varieties had two flowers in the cluster. The oldest 
varieties definitely known to bear more than two flowers on a stalk are 
Invincible Scarlet (1865) and Crown Princess of Prussia (1868). As 
these antedate the era of hybridization it is probable that the increased 
number arose by mutation. 

Novelty forms have also arisen from time to time. In double sweet 
peas there are two standards instead of one. In some varieties this 
character has been fixed by selection so that most of the flowers come 
double. It gives the eff"ect of increased size (Fig. 122), In the snap- 
dragon type of flower (Fig. 122) the standard is folded around the wings. 
It is recessive to en^ct standard and gives a simple Mendelian ratio of 3 
erect to 1 snapdragon in Fg. 



308 



GENETICS IN RELATION TO AGRICULTURE 



Habit in Sweet Peas. — There are several distinct types of plant in the 
sweet pea the origin of which may be definitely ascribed to mutation. 
The first Cupid plant (Fig. 123a) appeared among plants of the tall, 



'^^i'SS^WV. 



^^^s^m 



-^i 













Fig. 123. — a, Cupid or prostrate, dwarf sweet pea; b, bush or erect, tall form; c, Cupid X 
bush Fi, the ordinary tall form (folded over in order to photograph). (From Bateson.) 

white-flowered variety, Emily Henderson, in 1893. The growers, C. 
C. Morse & Co. of San Francisco, raised seven acres of the new variety 
in 1895 and every plant was true to type. This mutation has since oc- 




FiG. 124. — Dwarf or Cupid sweet peas. I, ordinary or prostrate Cupid; II, erect Cupid, the 
F2 double recessive from bush X Cupid. (From Bateson.) 

curred in a number of widely separated localities. The bush type also 
originated as a mutation from the tall form. The investigations of 
the factor relations of bush and Cupid sweet peas have been described in 



ON VARIETIES IN PLANTS 309 

a previous chapter. Scuui-dwarf, early-flowering sports have appeared 
even more frequently than those of the Cupid type. They have been 
made the basis of the winter-flowering types of sweet peas. Ordinary 
sweet peas pass into a semi-dormant condition for a time after germination, 
growing very slowly until sideshoots have been developed. The winter- 
flowering sorts, however, promptly send up a central axis which begins 
blossoming as soon as it has attained a height of from two to four feet. 
The Blanche Ferry group of varieties apparently had their inception 
in a mutation of this sort which a woman in northern New York noticed 
among some plants of the Old Painted Lady. She selected them for 
about twenty-five years after which they passed into the hands of a 
seedsman. From this stock a series of early flowering mutations have 
arisen in the order shown below. Black-seeded varieties are indicated 
by (6) and white-seeded ones by (iv). 

Old Painted Lady (h) 

I 

Bright-flowered sport {b) 

(30 years later) Blanche Ferry (/>) 

Extra Early Blanche Ferry (b) Emilv Henderson (white, w) 

I ■ I . 

Earliest of all (b) Mont Blanc (early white, w) 

I . , I . . 

Extreme Early Earliest of all (b) Earliest Sunbeams (primrose, iv ) 

I 
Earliest White (6) 

Fig. 125.— New varieties of sweet peas which originated by mutation among the progeny 

of Old Painted Lady. 

Hybridization and Selection in Sweet Peas. — The era of extensive 
hybridization in sweet peas dates from about the year 1880, consequently 
we can say but little of definiteness after that time with respect to the 
origin of new factors in the sweet pea save in a few particularly favorable 
cases. Laxton's Invincible Carmine was the earliest recorded new variety 
which was produced by crossing, and its parents are reputed to have been 
Invincible Scarlet and Invincible Black. We can easily understand, 
therefore, how it originated, for it is apparently merely an improved form 
of Invincible Scarlet resulting from the inclusion of the factor for intense 
pigmentation of Invincible Black in the factor complex of Invincible Scar- 
let. Similarly by hybridization it has been found possible to establish 
families of varieties such as the Spencer, the hooded, the grandiflora, 
and the winter-flowering sorts. Hybridization has throughout been 
merely a means of fully utilizing germinal differences which have arisen 
by mutation. It is true that in most cases we cannot say just when the 



310 



GENETICS IN RELATION TO AGRICULTURE 



particular features of form, color, and habit have arisen but we know 
that there was only one original form, and fragments of the history 
(Beal and Hurst) are sufficiently clear to give us assurance in advanc- 
ing this explanation of the role of hybridization in the creation of varieties 
of sweet peas. There is no authentic instance of a variety having origi- 
nated from hybridization of the sweet pea proper, Lathyrus odoratus, 
with any other species of Lathyrus, consequently^ the possibility of such 
germinal diversity is precluded. Similarly in the case of selection for 
more obscure characters such as number of blossoms in the cluster, size 
of flower, and vigor of growth, apparently the things that have been 
utilized in cases of improvement are mutations and new combinations of 
mutant factors. 




Fig. 126. — Four types of rose: a, typical modern Hybrid Tea rose, b, typical Hybrid 
Perpetual rose; c, the Damask rose, which was popular in old gardens; d, the old single 
Rosa gallica. (Reproduced from The Garden Magazine by permission.) 

Creation of Varieties of the Rose. — No finer examples of the origin 
of horticultural varieties by means of hybridization could be found than 
the garden roses of today. The genus Rosa is widely distributed in the 
Northern Hemisphere and contains several hundred species of which, 
according to Wilson, twenty-six have been utilized in the production of 
our garden roses. But these twenty-six species fall into fifteen distinct 
groups, and in habitat they represent Asia, Europe, and North America. 

The most important group of modern roses are the Hybrid Teas for 



ON VARIETIES IN PLANTS 



311 



tliey include garden and forcing varieties which combine marvellous 
beauty of form and color with vigor and hardiness (Fig. 126a). Four 
or possibly five distinct species enter into the ancestry of the group, as 
shown by the following pedigree. The Hybrid Ferpetuals (Fig. 12G^) 
are of mixed ancestry, all being hybrids of the Damask Rose (Fig. 126c) 
crossed either with Hybrid Bourbon or Hybrid Chinese varieties. 

The hardy, disease-resistant Japanese species, Rosa rugosa and R. 
wichuriana have entered into the ancestry of some of the best modern roses. 
Thus, the American Pillar variety is a hybrid between a red Hybrid 
Perpetual crossed with a hybrid between R. wichuriana and R. setigera, 
the Prairie Rose of America. Again, the Silver Moon variety is a result 



. Teas derivatives of. 



. Rosa chinensis var. odoratissima 



Hybrid , 
Teas . . . 



Hybrid 
Perpetual si 



Hybrid 
Chinese 



Hybrid 
Bourbons 




\Rosa damascena 
(Damask Rose) 



Fig. — 127. Pedigree of the hybrid tea roses. 



Rosa gallica 
(French or Provence 
Rose). See Fig. 126f/. 



Rosa chinensis 
(Chinese Monthly 
or Bengal Rose). 

Rosa centifolia 
(Cabbage Rose). 



of crossing R. IcBvigata, the Cherokee Rose, with a hybrid between the 
Tea Rose, Devoniensis, and R. wichuriana. These examples will serve 
to illustrate the composite ancestry of our best roses. The practicability 
of this method of procuring new varieties has of course been enhanced 
by the possibility of vegetative propagation. Occasionally valuable 
varieties have arisen as bud mutations but these usually differ from the 
parent variety only in some definite character, like flower color or habit 
of growth. 

In passing it is of interest to note how extensively this method of 
variety creation has been used by horticulturists, particularly in species 
which are normally propagated by clonal multiplication. The hybrid 
varieties of the rhododendron rival in diversity and floral magnificence 
even those of the rose, and like them they have been derived from the 
mingling of a number of different species. But it is among the Rosaceae 
particularly that horticulturists have found the most favorable subjects 
for hybridization. It is necessary in this connection merely to mention 
such familiar examples as varieties of plums, apples, strawberries, and 
other rosaceous fruits in the production of many of which extensive 
hybridization has been employed. In seed plants, also, there are many 



312 



GENETICS IN RELATION TO AGRICULTURE 



examples of like improvement. Unquestionably the amateur plant 
breeder can find no more fascinating or productive line of activity than 
that of selecting and working with some particular group of species from 
this standpoint. 

Origin of Varieties in the Boston Fern. — In 1915 Benedict reported 
that he had accumulated about 40 different forms of the Boston Fern, 
all of which had originated so far as is known from bud sports. The 
following statements regarding the source of these new varieties are 
based on Benedict's account. The original Boston Fern arose as a bud 
mutation from the tropical species, Nephrolepis exaltata. It was first 




Fig. 128. — 1. The original Boston fern, Nephrolepsis exaltata bostoniensis; 2, the first 
bud sport from the Boston, A^. exaltata bostoniensis Piersoni; 3, the Pierson fern next pro- 
duced elegantissima; 4, N. compacta, a sport from elega^itissima. {Courtesy Brooklyn Botanic 
Garden.) 

recognized as different from exaltata by F. C. Becker of Boston, and 
in 1896 it was named N. exaltata var. bostoniensis. The typical form of 
the species and the first sport, bostoniensis, are large growing ferns with 
uni-pinnate leaves (Fig. 128, 1). In the remarkable series of bud muta- 
tions that have been derived from bostoniensis within two decades, the 
principal characters undergoing transformation are, first, form of pinna 
and hence form of frond; second, size of frond; third, form of frond con- 
sidered independently of pinna-form; fourth, color of foliage. 

The original sport from the Boston fern was bi-pinnate; i.e., each 
pinna was subdivided into little pinnae or pinnules (Fig. 128, 2). This 
form appeared about 1900 in the establishment of F. R. Pierson of 
Tarrytown on the Hudson, and was named Piersoni or Tarrytown fern. 
It did not produce satisfactory plants because only part of the fronds were 
bi-pinnate; the remainder resembled the original Boston variety. 



ON VARIEriES IN PLANTS 



313 



But Piersoni soon produced a tri-pinnate sport which was more regularly- 
divided. Its fronds were somewhat shorter and much broader at 
the base, thus making the plant more compact. It was named elegantis- 
sima (Fig. 128, 3). Although it was unstable like Piersoni, its uniformity 
was considerably improved by selection. Soon it produced a sport of 
quite similar characters except that it was more dwarf which was named 
compada (Fig. 128, 4). In both elegantissima and compacta there was 
variation from the tri-pinnate to the quadri-pinnate condition. 

The Pierson fern also gave rise to another interesting series of new 
forms which exhibited variation in two more characters. In the ele- 




FiG. 129. — The fronds of modern commercial varieties differ greatly from those of the 
original Boston fern. The varieties shown here are relatively stable, although they are all 
likely in turn to produce new sports some of which may prove valuable, a, viridissima; b, 
Millsii; c, muscosa; d, vcrona; e, tnagnifica; f, superbissima. (After Boshnakian.) 

gantissima series the color of the foliage is similar to that of the original 
Boston form, but in the new sport, which was named superbissima (Fig. 
129/), the fronds are not only shorter and the pinnae three- or four- 
divided, but the foliage is of a deeper green color. Moreover, the fronds 
and separate pinnae are twisted so as to give the individual frond an 
irregular appearance although an entire plant appears fairly symmetrical. 
Although superbissima was unstable, producing uni-pinnate fronds occa- 
sionally, it soon produced a sport that is more compact in form and which 
proved to be more stable. This was named muscosa (Fig. 129, c). 

Other distinct uni-pinnate forms that have sprung as bud mutations 
either directly or indirectly from the Boston fern are the dwarfs, such 
as Scotti, Dwarf Boston, and Teddy Jr., and the vigorous, broad fronded 
variety, Roosevelii. There is no regularity in the production of larger and 



314 



GENETICS IN RELATION TO AGRICULTURE 



smaller forms. That is, a dwarf form may spring from a large form or 
from another dwarf form as shown in Fig. 130. Another distinct group 




Fi(i. 130. — Bud mutations in sports of the Boston fern. At the right ih) is the form, 
magnifica, a dwarf, asexual descendant of the variety, bostoniensis. The fern in the center 
(a) is a sport from this dwarf. It has a tendency to produce further sports and so could not 
be depended upon to breed true. At c is shown a small plant whose single frond resembles 
magnifica. At d is another sport that already displays instability in having two sorts of 
fronds. {After Boshnakian.) 




Fig. 131. — A series of pinnse illustrating progressive variation in division. 1, Var. bos- 
toniensis; 2, Piersoni; 3, Whitmani; 4, Goodi (or gracillima) ; 5, Magnifica; 6, Craigi; 
7, Amerpohli. (Courtesy Brooklyn Botanic Garden.) 



contains the more delicate, open, lace-Uke forms, such as Millsii and 
verona (Fig. 1296, d). The latter has an advantage over several earlier 



ON VAlilETlEH IN PLANTS 



315 



varieties of this group in that its raehis is strong enough to support the 
fully developed frond. 

As Benedict has shown. the bud mutations occurring in these ferns 
are more commonly regressive (showing more resemblance to bostonien- 
sis than to their parent forms), Ijut progressive mutations are found 
from time to time. These progressive changes take place along three 
main lines, viz., increase in leaf division (see Fig. 131), increase in ruffling 
or crisping, and dwarfing (see Fig. 132) ; and any form which has not 




Fig. 132. — A series of fronds illustrating progressive variation in ruffling and dwarfing. 
1, N. exaltata; 2, var. bostoniensis ; 3, Harrisi (or Roosevelti) ; 4, Wm. K. Harris (or new sport 
of Roosevelti) ; 5, Teddy Jr. ; 6-8, dwarf sports of Teddy Jr. ; 7, Randolphi. {Courtesy 
Brooklyn Botanic Garden.) 

reached the limits of possibility in variation along the first and last 
mentioned lines, may be expected to give rise to new forms showing 
further progressive variation in one or both of them. 

That these new varieties are produced by mutations in specific factors 
is indicated by the independence of character changes in series of suc- 
cessively produced forms that differ in several characters; for example, 
the appearance of dwarf uni-pinnate forms as sports of dwarf multi- 
pinnate forms. Various series derived from bostoniensis show progressive 
degrees of reduction in size of frond. When a dwarf tri- or quadri- 
pinnate plant throws a uni-pinnate sport the latter retains the dwarf size 
of its parent. Again the difference between Piersoni and superbissima, 
its sport, consists of the deeper color and twisted, irregular shape of 
the latter. When it in turn produced viridissima the new uni-pinnate 



316 GENETICS IN RELATION TO AGRICULTURE 

form retained the other distinctive characters of its parent. Finally, 
as Boshnakian points out, similar sports have been secured among sexually 
produced progeny in other species of Nephrolepis. Thus it appears 
that these interesting and valuable ornamentals owe their origin to altera- 
tions in specific genetic factors, i.e., to factor mutations in vegetative 
reproduction. 

We have found that new varieties of cultivated plants may be arti- 
ficially produced in either of two ways, viz., by the discovery and pre- 
servation of mutations or by hybridization. Factor mutations occur in 
both sexually and asexually reproduced plants and frequently produce 
new forms of immediate economic value. Sometimes, however, the 
original mutation may be merely a starting point indicating the line 
along which selection must work. There is always the possibihty that 
subsequent mutations in the same direction, even though they be 
minute, will be taken advantage of by the breeder. In the creation of 
new varieties for special purposes hybridization must usually be employed. 
The success of breeders in combining the desirable qualities of several 
species in the best modern varieties of the rose suggests untold possi- 
bilities in this field of plant breeding. 



1 



CHAPTER XVII 
THE COMPOSITION OF PLANT POPULATIONS 

Before taking up in detail the various methods of plant breeding and 
considering their effectiveness it is well to enquire as to the nature of the 
populations with which we are required to deal. By a population in this 
connection we ordinarily mean a variety as that word is used in the trade, 
although populations as found in cultivation may be made up of mixtures 
of varieties. Usually, however, within an established variety, that is, a 
strain or race bred to a given type until it reproduces that type with a 
fair degree of accuracy, the variations are of minor consequence and not 
always readily detectable. But they may be due not only to modifications 
consequent upon slight differences surrounding the development of 
individuals in a population; they may also be germinal, that is, they may 
arise either from Mendelian recombination of germinal differences or 
by actual new germinal changes. We desire to know, therefore, what 
sorts of populations exhibit germinal diversity, what kinds of germinal 
diversity they exhibit, and how the germinal diversity may be related 
to other characteristics of the populations. 

Reproduction in Plants. — ^In seed plants the important factor which 
determines the character of the population is the kind of pollination 
which normally takes place. In the following classification most of our 
important agricultural crop plants are listed roughly with respect to 
this factor. 

A. Plants normally self-fertilized. 

(a) Flowers hermaphrodite, but the floral mechanism such as practi- 
cally to preclude cross-pollination. Examples: wheat, oats, barley, rice, 
beans, peas, and most of the other legumes. 

(6) Flowers hermaphrodite, but the floral mechanism favorable to a 
low percentage of cross-fertiHzation. Examples : cotton, tobacco, tomato, 
flax, and other plants having a similar floral structure. 

B. Plants normally cross-fertilized. 

(a) Flowers hermaphrodite, self-fertile, but with floral devices favor- 
able to cross-fertilization. Examples: rye, sugar beet. 

(6) Flowers hermaphrodite, but self-fertiUzation precluded on account 
of self -sterility of the plants. Example: sunflower, red clover. 

(c) Monoecious plants, self-fertile, but the floral mechanism such 
as to favor cross-fertiUzation. Examples: maize, watermelon, squash, 
pumpkin, cucumber, and cantaloupe. 

317 



318 GENETICS IN RELATION TO AGRICULTURE 

(d) Dioecious plants. Flowers of different sexes on different plants, 
thus insuring cross-fertilization. Examples: hemp, hops, asparagus and 
date palm. 

Another class having hermaphrodite and uni-sexual flowers on the 
same plant is termed polygamous. The sunflower might be classified 
here, because its marginal ray flowers are pistillate only. Certain species 
of Compositae have the marginal flowers pistillate, through complete sup- 
pression of the anthers as in the sunflower itself, and the disk flowers are 
hermaphrodite, but the pistil always aborts, so that in effect they are 
really monoecious plants. In some cases, however, they are known to be 
completely self-sterile, so that cross-fertilization must always take place 
in seed formation. 

The above classification requires numerous qualifications. For ex- 
ample, it has been our purpose to list under Class Aa those plants which 
are so generally self-fertilized that it is not necessary to protect them to 
insure self-fertilization, but there are some species and varieties among 
them which sometimes exhibit a significant amount of cross-fertilization. 
The cultivated varieties of wheat are very rarely cross-fertilized, but the 
wild wheat of Palestine has a floral mechanism especially designed for 
cross-fertilization. Some varieties of rice, also, are cross-fertilized often 
enough in mixed plantings to make it impossible to assume self-fertili- 
zation in a given selection. In peas and beans, perhaps, the proportion of 
crossing is greater than in the cereals mentioned above, and in some cases 
it is absolutely necessary to protect them from insect activities. Thus Pearl 
and Surface in breeding investigations with Yellow Eye beans found it 
necessary to enclose selected plants in large muslin cages in order to 
exclude bumble bees, which were found to be effective enough agents 
of cross-pollination in open fields to disturb results greatly. On the 
other hand, however, Pearl and Surface in extensive investigations in 
oat breeding report not a single case of natural crossing. Also Rimpau, 
who carried on extensive investigations with nineteen varieties of oats 
over a period of six years, observed only five cases of spontaneous hy- 
bridization. Furthermore in most of the commonly cultivated varieties 
of wheat, barley, and rice natural crossing is so rare a phenomenon as 
to be worthy of special note in any observed case. We recall also 
Johannsen's pure line investigations with Princess beans which would 
have been impossible had natural crossing occurred among them in any 
significant amount. 

Among plants having hermaphrodite flowers which are usually self- 
fertilized there is also vast difference in the relative proportions of self- 
and cross-fertilization. In cotton, Balls has found it necessary to allow 
for about 5 per cent, of natural crossing. In tobacco self-fertilization 
is the rule, but it is not sufficiently assured to obviate the necessity for 



THE COMPOSITION OF PLANT POPULATIONS 319 

protection in gathering pure seed. Especially is this true in sub-tropical 
regions where humming })ir(ls are prevalent for they find tobacco flowers a 
splendid source of sustenance and unquestionably often effect cross- 
fertilization between plants. Moreover these remarks concerning tobacco, 
although they apply to the commercial varieties, do not indicate the 
true state of affairs in all species of Nicotiana, for a few species are 
completely self-sterile. Thus in N. alata grandiflora some individuals 
are actually completely self-sterile and others exhibit no bar whatever to 
seJf-fertiUzation. It is especially important, therefore, in dealing with 
plants in this class to determine these data for the particular species and 
varieties and the special conditions attending the experiments. 

Under Ba we have included rye in spite of general statements as 
to its self-fertility. This classification appears to be justifiable in view 
of reports of von Riimker and Leidner on results of inbreeding rye. 
The diflftculties in the self-fertilization of rye appear to be technical ones, 
rather than physiological, consequently reports as to its self-sterility must 
be in error. This is of interest in connection with the next following class 
which includes plants which are self-sterile. We have already mentioned 
the case of Nicotiana alata grandiflora in a given population of which 
both self-fertile and self-sterile individuals may be found. Other 
complications arise from contradictory reports as to self -sterility in some 
species belonging in these two groups. Thus there are reports that 
flowers on a given plant are sterile with their own pollen, but exhibit a 
certain degree of fertility when polhnated from some other flowers on 
the same plant. In effect such relations give results which are equivalent 
to self-fertility, but in some breeding operations it is important to know 
the exact relations, because it may be necessary to take advantage of them 
in special cases. It is probable that in general any difference which may 
be found in the fertilizing power of pollen derived from different flowers 
on a given plant are non-essential, and dependent upon some such 
factor as relative maturity of pollen with respect to the receptive 
period of the stigma. 

Among plants which are self-sterile are included a large number of 
the horticultural varieties which are normally propagated by means of 
clonal multiplication, but in which suitable pollination is necessary for 
fruit-setting or for the fullest abundance of fruit-setting. Orchard 
planting methods provide for this by mixing varieties which are known 
to act as efficient interpollinating agents. It is important to note that 
something more than a mere mixing of varieties is necessary; for the 
best results accurate knowledge should have been gained beforehand of 
the particular varieties which are most effective when planted together. 
Self-sterility in improved tree and bush fruits is a not unimportant con- 
sideration in practical horticultural operations. It is, also, of interest 



320 



GENETICS IN RELATION TO AGRICULTURE 



to note in passing that there is a possibihty in particular cases of dis- 
covering and overcoming the bars to self-fertiHty which are normally 
operative in such cases. 

Populations of Plants Normally Self-fertilized. — Continued self- 
fertilization in a population normally results in the automatic elimina- 
tion from it of all heterozygous individuals. The operation of this 
principle can be seen very clearly by considering the simplest case, a 
heterozygote for one pair of factors self-fertilized through a number of 
generations. Thus we see from Table XL VI that the general expression 
in this case for the percentage of heterozygotes after n generations of 
1 



inbreeding is 



2"" 



If we set this value equal to 1 per cent., we get 
2" = 100, n = 6.64 + . 



Accordingly beginning with a population made up entirely of individuals 

heterozygous for one pair of 
Table XLVI. — Proportioxs of Dif- 
ferent Genotypes and Percentages 
OF Heterozygotes in a Population 
OF Self-fertilized Plants 



Generation 


AA 


Aa 


aa 


Percentage of 
heterozygotes 







2 




100.0 


1 


1 


2 


1 


50.0 


2 


3 


2 


3 


25.0 


3 


7 


2 


7 


12.5 


4 


15 


2 


15 


6.25 


5 


31 


2 


31 


3.125 


n 


2" - 1 


2 


2" -1 


1 

2" 



factors, it would take only seven 
generations of inbreeding to 
reduce the proportion of hetero- 
z^'gotes within the population 
below 1 per cent. As a limiting 
value such a population would 
of course consist of 50 per cent. 
A A and 50 per cent. aa. 

Jennings and others have 
given generalized formulae for 
determining the percentage of 
heterozygotes where any num- 
ber, m, of pairs of heterozygous 
factors is involved in the 
original population. Thus starting out with a single plant having m 
pairs of heterozj^gous factors, or a population consisting wholly of 
such plants, the value for h, the proportion of heterozj^gous individuals, 
is given by the expression: 

This expression is very useful for determining the degree of homo- 
geneity which a hybrid population may be expected to exhibit after a 
given number of generations of self-fertilization. Thus assuming that 
there are 10 pairs of factors in a given cross, what proportion of hetero- 
zygotes will there be after five generations of sowing? The formula is 



THE COMPOSITION OF PLANT POPILATIONS 321 

Solving wc obtain h = 0.27; in other words, tlic clumces arc only 
about one in four that a plant selected from a population of this kind 
will be heterozygous. If there are 100 pairs of factors and ten generations 
of self-fertihzation only 9 per cent, of the population will be heterozygous. 
Thus we see how powerful is the tendency of self-fertilization to reduce 
the population to a homozj^gous condition. 

The number of homozygous genotypes to, which the population will 
be reduced, it should be remembered, is given by the expression, 2"', in 
which )n again is the number of pairs of heterozygous factors. If there 
are 10 pairs of heterozygous factors in the original individual, then the 
population will ultimately be reduced to 1024 different homozygous 
genotypes; if there are 100 pairs of such factors, the num])er of different 
kinds of genotypes is approximately 1,267,666 X 10-^. 

We should always remember in working with formula such as these 
that the}^ are only valid for conditions postulated in the premises. For 
the above formulae the following conditions are assumed: roughly 
equal viability of all genotypes, absence of any natural selection, and 
independent segregation of factors. Obviously none of these condi- 
tions is fulfilled in any even moderately complex population. We have 
already considered many examples of different viability in diverse geno- 
types, of which the many different Drosophila mutants provide the most 
conspicuous examples. Similarly natural selection of necessity enters in 
whenever any differences whatever exist in the ability of different geno- 
types to survive and reproduce themselves under a given set of condi- 
tions. In addition to these two obvious difficulties the universal 
occurrence of linkage also profoundly disturbs the mathematical rela- 
tions whenever any considerable number of factors is concerned in a 
given cross. It would be a very rare occurrence for even ten different 
pairs of factors to exhibit independent assortment in any plant species, 
impossible in a species like wheat which has but eight pairs of 
chromosomes. 

The biological significance of this mathematical discussion is merely 
this: that it demonstrates that populations in which self-fertilization 
is an invariable condition in seed formation must consist entirely of 
pure lines, if left undisturbed for a very few generations. Mathematic- 
ally the limiting condition is one in which all possible pure lines exist 
in constant proportions in the population, but biologically the limiting 
•condition is one in which the population is composed only of the most 
vigorous and productive pure lines. 

Populations as Affected by Crossing. — When a certain amount of 
natural crossing occurs the relations above described are somewhat 
disturbed. The population, of course, tends to reach an equilibrium, 
and for all practical purposes does reach one very soon, but the mathe- 

21 



322 GENETICS IN RELATION TO AGRICULTURE 

matical relations are much more complex than those given above. We 
may consider a simple case, however, and show the relations in that case. 
If we start out with a population consisting of equal numbers AA and aa 
forms, and assume that a given percentage of crossing occurs, then an 
equilibrium will be reached when the number of homozygotes produced 
by the heterozygotes in the population is equal to the number of hetero- 
zygotes produced by spontaneous crossing. Thus, if we assume 10 
per cent, of spontaneous crossing in such a population, in the first gen- 
eration of the 10 per cent, of AA which cross with other plants, half will 
be fertilized by other AA plants and half by aa. The latter will give 
heterozygotes, consequently the proportions of different genotypes 
produced by the A A plants will be 0.95^ A : 0.05 Aa. Similarly aa plants 
produce 0.05Aa: 0.95aa, so that in the first generation the ratio is 0.95AA : 
O.lOAa : 0.95aa. Now in the next following generation if we assume 
that random mating occurs among the 10 per cent, of plants which 
cross with other plants, then one-third of the plants in each genotype will 
mate with the same genotype, one-third with one of the other two geno- 
types, and one-third with the remaining genotype. That is, of the 
0.95AA one-tenth or 0.095 cross, as follows: ^AA X AA ^ 0.32AA, 
}iAA X aa =^ 0.032Aa and }iAA X Aa ^ 0.016AA : O.OlGAa. Simi- 
larly, of the 0.95aa, 0.095 cross: }-iaa X aa = 0.032aa, }-iaa X AA = 
0.032Aa and ^aa X Aa = O.OlGAa : 0.016aa. Also of the O.lOAa, 
one-tenth or 0.01 cross: }iAa X AA = 0.0016AA: O.OOlGAa, ^Aa X 
aa = 0.0016Aa : O.OOlGaa and }4Aa X Aa = 0.0008AA : O.OOlGAa: 
O.OOOSaa. Summating like genotypes we have 0.05AA : O.lOAa : 0.05aa. 
The 90 per cent, of AA and aa plants which are self-fertilized produce 
0.855AA and 0.855aa respectively, while the 0.09Aa plants which are 
self -fertilized produce 0.0225AA : 0.045Aa : 0.0225aa. Combining 
these with the results of cross-fertilization we have the ratio for the 
second generation, 0.928AA : 0.146Aa : 0.928aa. Now the ratio of 
the proportion of homozygotes to the population in the first generation 
is of course 0.95 and in the second generation it becomes, 

0.928 + 0.928 



0.928 + 0.146 + 0.928 



0.927. 



The composition of the third, fourth and fifth generations and the ratio 
of the proportion of homozygotes to total population for each are shown 
in Table XL VII. It is evident that, under the conditions assumed in . 
this case, the rate of change in the ratio of homozygotes to the total 
population becomes very gradual after the first three generations, so 
that for practical purposes the population has reached a state of 
equilibrium in the fourth generation. In this generation the ratio of 
heterozygous dominants to the sum of the heterozygous and homozygous 



THE COMPOSITION OF PLANT POPULATIONS 



323 



Generation 


AA 


Aa 


aa 


Ratio x/tf* 


1 


0.95 


0.10 


0.95 


0.95 


2 


0.928 


0.146 


0.928 


0.927 


3 


0.919 


0.167 


0.919 


0.917 


4 


0.915 


0.175 


0.915 


0.913 


5 


0.914 


0.179 


0.914 


0.911 



dominants is O.IG +• In this or later generations, therefore, tlie chances 
of selecting at random a heterozygous dominant, assuming dominance to 
be complete, are about one in six. 

Table XL VII, shows the composition of the population with ref- 
erence to a single pair of factors, A and a, in the first five generations 
when there is 10 per cent, of spontaneous crossing, assuming (1) that be- 
fore crossing began there were equal numbers of AA and aa plants; (2) 
that among the 10 per cent, of plants which cross random mating occurs: 
(3) equal fertility and viability in all individuals. 

Starting again with a population of ^A and aa forms we find that, 
assuming 20 per cent, of crossing in this instance, other conditions being 
the same, the ratio of homozygotes to the whole population in the first 
four generations is as fol- 
lows: 0.90, 0.86, 0.845 and Table XLVII.— Composition of Population 

0.837; while the ratio of 
heterozygous dominants to 
the total dominants in 
the fourth generation is 
0.27. Hence, in this and 
later generations the chance 
of selecting a heterozygous 
dominant is about one in 
four. Again, with 50 per 
cent, of crossing the ratio 

of homozygotes to the whole population in the first four genera- 
tions is 0.50, 0.625, 0.649, 0.662; and the ratio of heterozygous 
dominants to the total dominants in the fourth generation is 
0.50 + , so that the chance of selecting a heterozygous dominant is 
one in two. In the same way the theoretical expectation for any 
particular amount of crossing may be calculated. It must be borne in 
mind, of course, that we have made no allowance for greater relative 
vigor and productivity in the heterozygous plants. However, the 
method illustrated may be utilized in working out similar problems in 
which the genetic relations are disturbed by such conditions as difference 
in viability or fecundity as well as for various amounts of crossing. 

This brief consideration merely suggests the possibilities of mathe- 
matical analysis of the composition of populations under assumed condi- 
tions. It must be clear, however, that such analysis as applied to a given 
set of conditions would be of very great value in conducting breeding 
investigations. But it should be remembered that reliable conclusions 
regarding any particular case cannot be derived from such analysis 
unless the more important controlling agencies at least have been so 
carefully investigated that their combined influence can be duly esti- 



X = proportion of homozygotes in the popula- 
tion; y = value of total population. 



324 GENETICS IN RELATION TO AGRICULTURE 

mated. On the other hand, the general principles derived from the 
mathematical study of the composition of populations are of universal 
application. These principles may be summarized as follows: 

1. (a) Continued self-fertilization tends to eliminate all heterozygotes 
from the population. 

(b) The number of homozygous genotypes to which a self-fertilized 
population will be reduced depends upon the number of pairs of factors 
involved. 

(c) Such a population after a few generations will consist entirely of 
pure lines. 

2. (a) With a given amount of natural crossing in the absence of any 
disturbing effects there will be an approximation toward a definite pro- 
portion of heterozygotes in the population. 

(b) Such a population approaches very nearly a condition of equilib- 
rium within a few generations. 

(c) Under the influence of disturbing elements the proportion of 
heterozygotes may be increased or decreased, but the condition of 
equilibrium will be rapidly approached if the disturbing elements remain 
fairly constant. 



CHAPTER XVIII 
SELECTION 

The oldest and most generally used means of plant improvement 
must continue to be the basic method in systematic plant breeding. 
Although selection is universally recognized as an effective method of 
breeding, yet all too long the prevailing ideas among empirical breeders 
regarding the way in which selection effects improvement and the reasons 
why selection sometimes fails in securing the end desired have been exceed- 
ingly vague. The confusion of thought concerning this matter which 
still exists among both scientists and laymen is largely due to a lack of 
clear understanding concerning the nature of variation. The variations 
upon which selection can be used effectively owe their origin either to 
mutations or to recombinations of genetic factors. On account of the 
differences in the composition of populations in various species of plants 
the effects of selection differ greatly in different crops. In order to employ 
selection most economically the plant breeder should understand the 
nature of the population with which he is working and the genetic prin- 
ciples underlying effective selection. It is our purpose in this chapter to 
set forth the principles of selection in both allogamous and autogamous 
species. 

Selection Methods in Maize Breeding. — The maize plant is highly 
variable and many different varieties and strains have been produced by 
selection. In most of the states where corn is grown extensively the 
experiment stations have published bulletins on corn improvement and 
the subject is discussed in more or less detail in various works on plant 
breeding. We shall merely consider here certain methods of maize 
selection in order to illustrate the principles involved and to compare 
them with methods used in other crop plants. 

Inbreeding in Maize. — Self-fertilization in maize results in marked 
reduction in vigor and hence in size of plant and production of seed. This 
was first discovered by Shull, who applied the pure line method in corn 
breeding, and from his results inferred that a field of maize consists of a 
collection of genetically distinct biotypes which may be isolated by in- 
breeding. East soon corroborated Shull's discovery and later East and 
Hayes summarized the results of inbreeding a naturally cross-fertilized 
plant substantially as follows: 

1. There is partial loss of power of development, causing reduction in 

325 



326 GENETICS IN RELATION TO AGRICULTURE 

the rapidity and amount of cell division. This phenomenon continues 
only to a certain point and is in no sense an actual degeneration. 

2. There is an isolation of biotypes differing in morphological char- 
acters accompanying the loss of vigor. 

3. The hereditary differences between these biotypes is often indi- 
cated by regression away from instead of toward the mean of the general 
population. 

4. As these biotypes become more constant in their characters the 
loss of vigor ceases to be noticeable. 

5. Normal biotypes with such hereditary characters that they may be 
called degenerate strains are sometimes, though rarely, isolated. 

6. It is possible that pure strains may be isolated that are so lacking 
in vigor that the mechanism of cell division does not properly perform 
its function, and abnormalities are thereby produced. 

Thus we know that any commercial variety of corn is a mixture of 
different genotypes and that inbreeding tends to isolate pure genotypes, 
i.e., inbred strains tend to become homozygous. Thus it is evident that 
the cross-bred progeny of two different inbred strains will be heterozy- 
gous for many factors. That cross-bred maize frequently displays greater 
vigor than either parent was first demonstrated by Beal of Michigan in 
1878. But it was not until Shull and East demonstrated the existence 
of genotypes in maize that the genetic significance of this phenomenon 
became evident. The actual cause of the increased vigor has been ex- 
plained in various ways. Both Shull and East held that decrease in vigor 
in inbred strains is due to reduction in the number of heterozygous 
factor combinations and that increase in vigor in Fi hybrids is the result 
of increase in the number of such combinations. The general occurrence 
of decrease in vigor upon inbreeding naturally cross-bred species and of 
increase in vigor upon crossing closely related forms led them to conclude 
that heterozygosis is the cause of increased physiological vigor in Fi 
hybrids. Other explanations of this phenomenon have been offered, 
one of which was that of Keeble and Pellew, to the effect that it 
"may be due to the meeting in the zygote of dominant growth factors 
of more than one allelomorphic pair, one (or more) provided by the 
gametes of one parent, the other (or others) by the gametes of the other 
parent." East and Hayes reject this hypothesis on the grounds that this 
increase in vigor "is too universal a phenomenon among crosses to have 
any such explanation. Furthermore, such interpretation would not 
fitly explain the fact that all maize varieties lose vigor when inbred." 
But there is good evidence that all maize varieties do not lose vigor to 
the same extent when inbred and that certain genotypes produce much 
more vigorous Fi hybrids when crossed than other genotypes. As was 
stated in Chapter XII, D. F. Jones has explained this increased vigor 



SELECTION 327 

in F, hybrids in tornis of dominance and linkage (p. 231, 2). The fact 
that different genotypes give diverse results when crossed is of immense 
practical significance. 

The Ear-to-row Method. — This has been the method of commercial 
corn improvement for many years and it is well illustrated by the IlUnois 
corn breeding experiments, which have been going on continuously for 
over 20 years. The original purpose of the experiments was to produce 
new strains which would be more valuable as a source of feed for hvestock. 
It was found that there was considerable variation in the relative amounts 
of protein and carbohydrates in the grains of different ears. Accordingly 
selection was begun with the object of increasing the protein and reducing 
the starch content of the grains; also of decreasing protein and increasing 
starch. As oil was worth three times as much as starch per unit of weight, 
selection for higher oil content was also begun, A low oil strain was 
started for comparison and such corn was soon found to be desirable for 
the production of pork and beef of high quality. 

The work was begun by Hopkins who picked out 163 ears of a local 
strain known as Burr's White, made a chemical analysis of a few grains 
from each ear, and on that basis sorted them into four classes, viz., high 
and low protein and high and low oil. The strains were grown in isolated 
plots from the beginning. After 9 years of selection it was found to be 
necessary to prevent inbreeding. Accordingly in the tenth and succeed- 
ing years about 24 ears were selected for each plot and one row was 
planted from each ear, then the even numbered rows were detasseled. 
Subsequent selections were made from the detasseled rows, the first 
consideration always being high yield. Usually 20 ears were taken from 
each of the six higher yielding rows, or 120 ears for each plot. These 
were tested by chemical analyses and the most extreme variants in the 
desired directions were selected for the next planting. 

The results in general have been more regular in the high and low oil 
series than in the high and low protein strains. In the latter there seems 
to have been no very decided effect of selection after the first 10 years. 
Similarly there has been no continuous advance in the low oil strain since 
the seventeenth year of selection, but in the high strain the per cent, of oil 
has continued to increase slightly. The progressive effects of selection in 
the four series are graphically illustrated in Figs. 133 and 134. That the 
striking results depicted in these graphs were not caused by environ- 
mental conditions was proved by planting mixed plots with two grains of 
''high" and two of "low" corn in each hill so arranged that the resulting 
plants could be identified. This test, according to L. H. Smith, was made 
for three successive years, and subsequent analyses showed that under 
these conditions the different strains maintained their distinguishing 
chemical characters. 



328 



GENETICS IN RELATION TO AGRICULTURE 



The Illinois Station experiments have included selection for many- 
other characters of the corn plant in more recent years. One of the most 
striking results was obtained by selecting for height of ear on the plant. 
Data on which to base selection were secured by measuring several 
hundred stalks in the oil and protein plots, noting height of ear above the 
ground, total height of stalk, apparent number of internodes below the 




'B6 '97 -as '99 '00 '01 '02 '03 '04 '05 'OC '07 'OS '09 '10 '11 '12 '13 'H '15 
Year of Selection 
Fig. 133. — Two graphs representing the effects of selection for high and low oil content in 
the Illinois Station corn experiments. {Data from Castle.) 

ear and number of internodes above the ear. Fig. 135 shows the result 
of selecting for high and low ears during five generations. Similar results 
were obtained from selection in the case of position of ear at maturity 
and total yield. 

The striking results of these carefully conducted experiments have 
been citfed by various authors as evidence par excellence for the most 




Fig. 134.- 



'96 '97 "98 '99 '00 '01 '02 '03 '01 '05 '00 '07 '03 '09 '10 'U '12 '13 '11 '15 
Year of Selection 
-Two graphs representing the effects of selection for high and low protein eon- 
tent in the Illinois Station corn experiments. {Data from Castle.) 



diverse conceptions of the rdle which selection plays in evolution and 
breeding. Thus the earlier allusions of Hopkins and Smith, the discus- 
sion in E. Davenport's text on breeding, and the recent treatment by 
Castle all seem to attribute a peculiar creative power to selection which 
meets with a certain "response" on the part of the plant. This is in line 
with the Darwinian idea that all fluctuating variations are heritable and 
that the continuous selection of minor fluctuations in a certain direction 
is always effective in shifting the type. 



SELECTIONS 



329 



The futility of attempting to generalize regarding the effects of selec- 
tion in plants must be obvious from what we now know about the com- 
position of plant populations. With the application of Johannsen's 
genotype conception in analyzing the composition of a field of maize the 
problem of explaining the role of selection in the Illinois corn breeding 
experiments was immediately simplified. This was perceived by ShuU 
who pointed out that the results of these experiments might be readily 
explained on the ground that some hybrid combinations of genotypes 
have greater capacity for the production of the desired qualities than 
other combinations, and that the selection has gradually l)rought about 




Fig. 135. — Result of selecting corn for high and low ears during 5 generations. The 
white tape marks the position of the ears on the front row of plants in both plots. 

the segregation of those genotype combinations which had the highest 
capacity for the production of the desired quality. 

Meanwhile Surface had made an illuminating analysis of the data 
from the first 10 years of selection as reported by Smith. This treatment 
is so valuable as to warrant its examination in some detail. At the time 
the selections were made a careful record of the pedigree of each ear was 
kept. These pedigrees are of course for the maternal side only since 
self-pollination was not practised. From these data Surface prepared a 
pedigree chart for each of the four strains. The chart for the high- 
protein strain is reproduced in Tables XL VIII and XLIX. As stated 
above 24 ears containing the highest per cent, of protein were selected for 
the 163 ears analyzed in 1896. These were given registry numbers from 
101 to 124 inclusive as shown in column one of the two tables. For 
convenience we may refer to these ears as the first generation of high- 



330 GENETICS IN RELATION TO AGRICULTURE 

Table XL VIII. — Pedigree Chart of High-protein Corn — Part I. {After Surface.) 



Generation Number 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


11 


101 












102- 215- 


320- 


410- 


502 






103- 208- 


314.. . 


424 
409 








104- 214..^ 


f 316- 
310 


421 








105 


306. . ■ 


401- 


514 






106.. {III. 


315- 
319.. 


405 
418 
417 
414 
416 








107.. {^^r 


301 










108.. {,206- 


321.. ■ 


415 
406 








109 




407 - 


510 






110 


311.. 


420 








111 


312 


411. . { 


506 - 604 
513 








212.. 


313 


404 


[614 






112. . 




309.. 


402 - 

408 
422 


515.. 606 
[603 








205- 


317.. 


412.. { 


503 












509 - 613 - 705 - 


- 822 




113 (210 
i^-^- • \220- 


324 










114- 204- 


303 










115- 224- 


304 










116- 202 












117 












118- 218- 


302 










119 (221- 

i^y.. (201- 


307 - 
305 


403 - 


511 






120- 203- 


308 











SELECTION 331 

Table XLIX. — Pedigree Chart of High-protein Corn — Part II. {After Surface.) 



Generation Number 


1 


2 


3 


4 


5 


6 


7 


8 




9 




10 


11 




'706.. 


r808. 
813 


/901 
I 908 






609 . . < 


[801 








^512.. 


[ 702 




'912. 


f 1007 

1002 

1 1014 

. 1019 






U08 










r612- 711 


.814. 


J 

1 




|'423 




601- 710.. 


810 

] 
821 


925 
1 920. 


f 1001 f Vm 

1020 -' ^'^ 
1013 ^ ""'"^ 






505 






f 916 




121(207 -323... 


413. 


501 


r 719 

fiO'? J 721 
i 712 


^ 811. 
'819 


902 
' 905 
.914. 


fl009 .J, 
J 1016 02 

1 ITl 1 1107 
I i'J^i ^ 1119 










906 

] 

[ 924 . . 










' 713.. , 


806. 
802 


[1017 .,. 
J 1024 Jjlf 
1 1012 -' ^^ 
005 1120 
^ ^"""^ [ 1108 




507. . 




704 


820. 


r903 

917 

[ 910 






508 


607 . . 






















1 


714 






' nil 
r 1010 j 1106 

1 1123 
[ 1118 






716- 


807 












/911 
\ 922.. 


1 






717.. J 


818.. 


1003 










1015 r ,,^r 




[ 


804 


f915 


( 1105 




605 / '^07 
605.. ^ 720 


'812. . 
805 


919 

[ 923 


1022, 1112 
1117 




f 708 


815 








709.. 




f 907 




I 


fill J 


803.. 


i 913 


' 1011 




on . . < 


703 


817 


t 904 
f 918.. 


1018 f {1^9 

1006. ^ }J?^ 

[ WZ.i ^ JJ21 


122 


I 701 1 


809. . 


\ 

[ 921 


123 










.c,, / 216 -318 

^^''•- \ 209- 322-419- 504- 


610 ^"^18- 


816- 


909 





332 GENETICS IN RELATION TO AGRICULTURE 

protein corn. The next season 4 sound ears were analyzed from each 
of the twenty-four rows. From these 96 ears the 24 again having the 
highest per cent, of protein were selected for planting. The distribution 
of these selected ears among the 24 original ears is shown in column two 
of the tables. For example, it is seen that ear No. 124 produced 2 
ears, Nos. 216 and 209, which were among the first 24 as regards protein 
content. Ear No. 123 on the other hand failed to produce any ear (so 
far as the ears analyzed showed) sufficiently rich in protein to be in- 
cluded among the first twenty-four. Thus 8 of the original ears fail to 
be represented in the second generation, while 8 other ears contributed 
2 ears each for planting the following year. Exactly the same selection 
was practised in the second year and the resulting selected ears are 
shown in the third column of the tables. Of the 16 original ears rep- 
resented in the second generation only one. No. 116, was dropped out 
in the third generation but in the next generation there is a significant 
dropping out of some of the original lines, so that in the fourth genera- 
tion only 9 of the original 24 ears are represented by progeny. Five of 
the original lines contribute 80 per cent, of this generation, while two 
lines, 106 and 112, contribute nearly 60 per cent. Hence at the end of 
the fourth generation it is clear that certain of the original lines have a 
much greater tendency to produce ears with a high per cent, of protein. 
By simply selecting on the basis of the protein content of the individual 
ear for 4 years 70 per cent, of the original lines have been dropped. 

Thus the elimination of the original lines gradually proceeds until, 
in the tenth and eleventh generations all of the high-protein corn is the 
offspring of a single ear, viz.. No. 121. It will be remembered that in the 
tenth year the method of detasseling alternate rows and saving seed from 
these only was put into effect. But this change in method could not 
have induced the results we have noted because line No. 121 had demon- 
strated its superiority over all the others as early as the seventh genera- 
tion. This isolation of a single line was brought about therefore simply 
by selecting each year those individual ears that showed the highest 
per cent, of protein. Starting with a protein content of 10.92 per cent., 
at the end of the third year (fourth generation, 1899) the protein content 
was only 11.46 per cent, or a gain of 0.54 per cent. But the next year 
(fifth generation) the protein content jumped to 12.32 or a gain of 0.86 
per cent, in 1 year. Referring now to Table XLIX it is seen that it is in 
1899 that a great reduction was made in the number of lines represented, 
for in the fifth generation only six of the original twenty-four lines remain. 
Furthermore it is just here that line No. 121 begins to show its superiority 
since 5 of the 15 ears selected in 1900 or 33)^^ per cent, come from this 
line. 

The course of events in the other three strains was similar but not 



SELECTION 



333 



Table L. — Genetic Relations Between Certain 
Physiological Characters of the Corn Grain. 



Character 



Dominant 



Recessive 



Moisture High 

Nitrogen and protein| Low 



Crude fat. 



quite so striking. In the low-protein strain only two of the twelve 
original lines are represented in the eleventh generation; in the high-oil 
strain three lines out of twenty-four are maintained throughout the 
10-year period ; and in the low-oil strain only two lines out of twelve are 
represented in the eleventh generation. 

These results are exactly what would necessarily accrue in any al- 
logamous species under continuous selection for a given character, pro- 
vided the degree of expression of that character is dependent upon a 
number of genetic factors. That several chemical characters of the corn 
grain, including protein and oil (fat), are inherited in accordance with 
Mendelian principles was determined })y Pearl and Bartlett in 1911. In 
a cross between a white 
sweet corn and a yellow 
starchy corn determina- 
tions were made by direct 
analysis of the percentage 
content of the grains of 
the pure parent races and 
the Fi and Fi progeny in 
respect to nine chemical 
constituents. These are 
hsted in Table L, which 
also indicates the dominant 
and recessive conditions of 
these characters in the 
cross studied. 

This evidence^ although 
worked out quite independ- 
ently, supplements Surface's analysis of the lUinois data in a remark- 
able way. Although there are technical obstacles to a clear cut de- 
termination of the factor relations' involved, yet there is no question 
whatever that these characters of high and low protein and oil are con- 
ditioned by unit factors. A priori there is no objection to assuming the 
existence of several factors which affect the percentage of protein, for 
example, and that the original ear, 121, of the superior Hne in the 
high protein strain represented a genotype rich in high protein factors. 
Similarly in the other strains, continual ear-to-row selection has 
gradually eliminated all genotypes except the one, two or three as 
the case may be of highest or lowest factor combinations. 

Thus we see that selection has created nothing in the course of these 
justly famous experiments; it has served merely as a means of isolatmg 
particular combinations of factors which condition oil and protein pro- 
duction in the corn plant. Moreover, this sorting process has not been 



Ash 

Crude fiber. 
Pentosans . . 



Sucrose . 



Dextrose . 
Starch. . . 



Low (incomplete 
dominance) 

Low 

Low 

Low (incomplete 
dominance) 

Low (incomplete 
dominance) 

Low 

High 



Low 

High 

High 

High 
High 
High 

High 

High 
Low 



334 GENETICS IN RELATION TO AGRICULTURE 

entirely regular or continuous. The saltations or jumps revealed by- 
Surface's analysis were directly consequent upon lump elimination of 
a number of mediocre lines. These results, therefore, are in entire har- 
mony with the known nature of allogamous populations. This conclu- 
sion is further corroborated by the recent report of Reitz and Smith on 
the statistical study of indirect effects of selection for high and low pro- 
tein and oil. These authors state: 

''It is found that four distinct types of corn as regards length, circumfer- 
ence, weight of ears, and number of rows of kernels on ears are so well estab- 
lished that we may assign orders of values to the means of these characters that 
persist with but a few exceptions in such changes of environment as have been 
experienced in 11 years of planting, from 1905 to 1915. 

"While a few slight but progressive changes have been noted, the selections 
for chemical composition from 1905 to 1915 have not changed decidedly the dif- 
ferences in mean values of these characters. In fact, we are unable to assert with 
any high degree of probability that the strains differ more or less with respect to 
these characters during the second half of the period 1905 to 1915 than during the 
first half." 

The italics are ours. It is of especial significance that careful biometrical 
study has failed to reveal any progressive change as a result of continued 
selection in these strains of corn. For the results of these experiments 
have been cited as evidence par excellence by Castle in support of his 
hypothesis of factor variability. 

The ear-to-row method has been modified in various ways but it 
still forms the basis of most systems of commercial corn breeding. A 
popular feature of systematic corn improvement is the use of score cards 
in judging. A special development of the score card method of selection 
is the use of selection index numbers as advocated by Pearl and Surface. 
In this plan arbitrary values are assigned to various characters of the 
corn ear, for example, such as absolute size of the ear, average percentage 
depth of the grains, etc. The idea is to combine in a single numerical 
expression the values of a series of variable characters with regard to 
all of which the breeder wishes to practice selection at the same time. 
The index numbers of different varieties are not directly comparable 
but for a given variety they may be useful as an adjunct of the score 
card method. However, their use requires more attention to details 
and hence greater expense than most breeders can afford to give. Their 
use in plant breeding will probably be limited to experiment stations 
(see Chapter XXXI). 

The danger of continued ear-to-row selection or "narrow breeding" 
within a variety was pointed out in 1909 by Colhns, who emphasized 
the importance of "broad breeding" in such crops as exhibit loss of 
vigor when closely inbred. About the same time Williams inaugurated 



SELECTION 



335 



the remnant system of corn breeding at the Ohio Experiment Station 
and the plan was adopted hy the Ohio Corn Improvement Association. 
The plan calls for an ear-to-row test plot each year in which ears are 
carefully tested for productiveness. Only half of the grains on each 
car are planted in the test plot, the remainder being retained until the 
following year under the term "remnant." The car-to-row test plot 
need not be isolated as no seed is saved from it. The next year the 
remnants of a few, usually four, of the highest yielding ears are planted 
in an isolated breeding plot, and the stalks from all of the ears planted 
in this patch, except those of the highest yielding ear, are detasseled. 
Seed ears are selected from the detasseled rows and grown the next 



ITf^^^lHi^^^^^HBI^^^^^Hil^H 






^^V^^T^H 








^^'^^ 


m^^M^^KW^^^^ 


\^M 


^m^^ 


^^Hil^l 


K 


wmMKmWm) 


nl 


n^^H 


^S/mk 


^H 


^B <- 






"^■1 


^^1 


- ^B 


1 ■ 




I il^H 


HH 


Hi 


1^1 


.1 rfiB^^^I 


■V 


11 




- ^H 


H jj^l 




»p 3|^^^^| 


MiWM 




■ r 


^H 1 




r^fl 


H't^^H 


I'iH 


Jl^^l 


^mJM 


ilH 


^m 


aH 


^^kuM^I 


H ^^^^^1 


^^^^^^1 



Fig. 136. — Delta Farm White Dent, a superior strain of maize adapted to the bottom 
lands of the interior valleys of California. It is the result of 30 years of continuous selection 
of seed in the field before harvesting. The original material consisted of a mixture of all 
the types of corn commonly grown at that time. A convincing demonstration of the prac- 
tical value of seed selection as a general agricultural practice. 

year in a multiplying plot to supply seed for general planting. After 
this method is under way on any farm, there is maintained on the farm 
each year a small isolated breeding plot, a multiplying plot, and an 
ear-to-row test plot. This method successfully excludes from the breed- 
ing plot all individuals except those whose producing power has been 
found to be very high. At the same time it provides for the intercrossing 
of these most productive strains, and by continuing the tests from year 
to year the work will "tend toward the selection of the best producing 
ears for all or average seasons." According to Hartley, "The choice 
of a high yielding variety is important; the choice of high yielding ears 
is even more important." The remnant system combines this result with 
the advantages attendant upon intercrossing of distinct strains. Rye, 
clover, beets, timothy and other grasses are suited to this method of 
breeding. It was with rye that Rimpau first employed the system 
that later came to be known as the German method of broad breeding. 



336 



GENETICS IN RELATION TO AGRICULTURE 



An interesting illustration of what can be accomplished in maize 
merely by mass selection, when a definite ideal is maintained and seed 
is selected in the field before harvesting, is found in the Delta Farm White 
Corn shown in Fig. 136. 

Selection Methods in Breeding Close-pollinated Plants. — The suc- 
cessful methods of breeding wheat have been reviewed in preceding 
chapters. Compared with the methods required for corn the work of 
isolating genotypes in wheat is relatively simple. Most commercial 



I l2 




Fig. 137. — Typical heads from seven pure lines of Defiance wheat. Nos. 1 and 2 do 
not yield one grain per spikelet on the average; Nos. 6 and 7 yield from 4 to 7 grains per 
spikelet. Note tendency to club type in No. 6. 

varieties of wheat are a mixture of pure lines which can be isolated by 
single plant selections. In Fig. 137 is shown a typical head from each 
of 7 different pure lines isolated by selecting single plants from a plot of 
Defiance wheat. Nos. 1 and 2 did not have an average of one grain 
per spikelet while Nos. 6 and 7 bore from 4 to 7 grains per spikelet. If 
Nos. 6 and 7 prove to be superior in other characters also, they need only 
to be multiplied in order to yield greatly improved strains of the Defiance 
variety. It was by this method that Roberts in 1906 isolated a pure 
line of Turkey wheat that appears very promising for the Great Plains 
Region. It is worthy of note that this superior pure line was the 135th 



SELECTION 



337 



single head selection made by Roberts in 1906. Altogether he made 557 
selections from nearly 200 different varieties; but nearly 415 of these were 
discarded within 2 j'oars. 

The Plant-to -row Method. — Single plant selections are usually grown 
in garden rows, each row from a different plant. Final selection of the 
individual plants should be preceded by field observations, noting habit, 
vigor, disease resistance, season of l)looni, time of maturity, productivity, 
etc. Each of these plants must be harvested separately and careful 




Fig. 138.- 



-Spreading and tTcct pure lines of Gypsy wheat, 1907. 
A.E.S.) 



{After Williams, Ohio 



records should be made concerning yield and other important characters. 
It is on the basis of the field observations and the data from the harvested 
plants that a further selection must be made. From each of the plants 
finally selected sufficient seed is taken for a row of about 25 plants. The 
rows should be evenly placed and plants should be equidistant in the row. 
By subjecting these rows to severe selection the future work may be 
considerably reduced. Hence careful notes should be taken throughout 
the season and at harvesting time. Of several hundred rows only a 
few may be found good enough to be continued. The third year the 

22 



338 



GENETICS IN RELATION TO AGRICULTURE 




Fig. 139. — The erect pure line of Gypsy wheat in 1909. (After Williams, Ohio A. E. S.) 




Fig. 140. — The same pure line (on the right) now known as the Gladden variety, as grown 
in 1915. (After Williams, Ohio A. E. S.) ^ 



SELECTION 



339 



seed from the selected rows is sown in small multiplying plots. At the 
Maine Station these plots are 1-2000 acre in area and usually in duplicate 
(see Fig. 141). These plots are subjected to still further selection and 
only the best retained. The next step is to sow each selected pure hne 
in one or more field plots. At Maine 1-40 acre plots are used and each 
line is tested in duplicate or quadruplicate plots for several years and 
only those that are superior in some respects at least to commercial 
varieties are retained. At the Ohio Experiment Station according to 
Williams, "In following the pure line method of selection, decided dif- 
ferences in winter resistance, stiffness of straw, yield of grain and bread- 




FiG. 141. — Planting board usecLin pure line work with small grains at the Maine Exper- 
iment Station. It provides a plot ^-2000 acre in area with the plants nearly as close together 
as when sown in the field. {After Surface and Zinn, Maine A. E. S.) 

making qualities have been found in the progeny of individual heads 
selected from ordinary varieties of wheat." In Fig. 138 are shown two 
■very distinct pure lines of Gypsy wheat as they appeared in 1907. In 
Figs. 139 and 140 the same pure line appears as grown in 1909 and 1915 
respectively. This selection, has been introduced under the name of 
Gladden. 

Ineffectiveness of Continued Selection Within Pure Lines. — Con- 
vinced of their failure to make any progress as a result of continued 
selection within pure lines, some experiment stations have abandoned 
this line of work. Hutcheson has reported on the results of 13 years 
of continuous selection in six pure lines that were isolated by Hays 
in 1901. These pure lines represent five of the sub-species of common 
wheat, Triticum vulgare. In brief the method consisted of selecting each 
year the best 100 grains from each of five or more best plants in each line. 
This seed was planted at regular distances in centgener plots the following 
year, each centgener representing a single plant selection. Hutcheson 



340 GENETICS IN RELATION TO AGRICULTURE 

says, "the indications are that from a practical breeder's standpoint 
permanent improvement in pure Hnes in small grains, if possible, is 
certainly not rapid or apt to be very marked." He also suggests that 
much more rapid progress could be made by isolating pure hnes from 
mixed populations and combining the desirable characters of these lines 
by hybridization. 

Other crops in which the method of selecting pure lines is applicable 
are oats, barley, peas and beans. Notable improvement has been made 
in oats by this method at the Svalof, Cornell University and Maine 
Experiment Stations. The general method of procedure at the Maine 
Station is indicated by Surface and Zinn in their bulletin on pure line 
varieties of oats (see p. 371). The pure lines finally retained came 
from only three varieties, viz., Banner, Irish Victor and Imported Scotch. 
It is noteworthy and consistent with Mendelian principles that the 
physiological characters which result in higher yield are not necessarily 
associated with morphological characters in the plant or grain. Similar 
results with winter-resistant barleys have been reported by Spragg of 
the Michigan Station. The practical importance of the selection of 
pure lines as one phase of a complete system of breeding as practised 
with autogamous species is given further attention in Chapter XXI. 

Selection which is to result in the isolation of the most superior 
genotypes must begin with individual plants. In dioecious and self- 
sterile plants this method is inapplicable. Here the breeder must begin 
with phenotypically similar individuals and continue inbreeding of simi- 
lar plants for several generations in order to isolate approximately uniform 
strains. The earlier improvement of the sugar beet was accomplished 
by mass selection. But in recent years the producers of commercial 
seed have introduced a system of line selection. According to Briem, 
reliable seeds cannot be obtained by selection in the lump, nor from a 
single generation of mother beets followed by the cultivation of seed roots. 
An individual selection must be made the characteristics of which are 
assured by testing for three generations. That is to say, since the beet 
is a biennial 6 years are required to obtain seed of guaranteed quality 
for the seed roots and another 2 years must pass before the market 
product is ready. Briem's opinion is in harmony with Pritchard's con- 
clusion that continuous selection is not an efficient method of sugar beet 
improvement and that the improvement of the past is the result of 
isolating mutations (see p. 369). 

In emphasizing the importance of finding the best genotypes within 
a chosen species or variety the usefulness of mass selection should not be 
overlooked. It is frequently the first or only practicable step to take in 
purifying a commercial variety. The so-called "running-out" of varie- 
ties can be prevented by reasonable care to avoid mixing seed and by occa- 



SELECTION 341 

sional mass selection from the field. Seed selection of this sort is of the 
greatest practical value to agriculture and it is applicable to most sorts of 
field and garden crops. 

The Practical Importance of Keeping Varieties Pure. — Many farmers 
do not regard purity of varieties as a matter of great concern, but con- 
tinue to use impure seed from year to year. Since the main object of 
breeding work is to produce new and better varieties, and since a true 
variety differs definitely from all other varieties, it is of great impor- 
tance that its purity and hence its identity be maintained. The need for 
care in this regard is of course much greater in naturally cross-fertilized 
species than in self-fertilized forms, yet even in the latter the mixing of 
varieties may detract greatly from the market value of the crop. It is 
not impossible for an impure variety or a mixture of varieties to give good 
returns for a year or two or even longer. When one considers, however, 
the rapidity with which the number of distinct strains may be increased 
by occasional crossing the danger of such practice will be realized. For 
this reason all agencies supervising the collection of stock seed of com- 
mercial varieties of corn, sorghum, cotton, etc., should exercise every 
possible precaution against mixing varieties or collecting seed that may 
have been crossed with other varieties. As Newman points out, 
however, there are certain circumstances under which the planting of 
mixed sorts may have their advantages. Thus a variety may contain 
strains which differ from each other for example chiefly in their response 
to different soil conditions. Were a variety of such composition sown in a 
field in which the soil is exceedingly variable it is possible that a better 
average would be maintained than from an absolutely pure sort which 
demands more exact conditions. Yet even here the practice marked 
of careful mass selection in the field would doubtless result in marked 
improvement. In general, however, the diflficulty of knowing the real 
nature of the strains which compose a mixed variety makes it unsafe 
to depend upon the possible virtues of maintaining the most advantageous 
mixture. Proved sorts of general adaptability offer much greater 
promise. 



CHAPTER XIX 

HYBRIDIZATION 

The usual purpose of the plant breeder who resorts to hybridiza- 
tion is to secure new and better combinations of characters among the 
progenies resulting from his crosses. Improvement of a given species 
may consist merely in the eUmination of undesirable characters or of the 
production of entirely new combinations of characters already existing 
within the species. In this work the apphcation of the Mendehan 
principle of segregation and recombination is of the greatest prac- 
tical value. By concentrating his attention on only a few important 
characters at a time the breeder can sometimes secure the desired 
combinations in F2. But at the same time one who is informed in 
regard to modern genetical principles will be prepared for possible 
disappointment in meeting an early realization of his aim. Moreover, 
he will understand how to select in F2 and later generations for further 
testing. He will realize that a specific character difference in his parental 
forms may be conditioned by more than one factor difference; also that 
some specific factors display considerable variabihty in expression; and 
that hnkage, crossing-over, multiple factors and multiple allelomorphs 
may play a role in conditioning or preventing the particular character 
combination for which he is striving. Furthermore, the ideal sometimes 
demanded of the breeder involves character complexes which include all 
the functions of the plant. As has been shown already the comparative 
difficulty between different cases of this sort depends directly upon the 
number of chromosomes possessed by the species in question. Finally, 
demands are sometimes made for the "creation" of characters which are 
unknown in available phenotypes and for which there is no genotypic 
representation within the species. In such cases recourse may be had 
perhaps to species hybridization. But those who are famihar with the 
results of species hybridization will be prepared for complete disappoint- 
ment from the first. It is not the motive of these remarks to discourage 
intending hybridizers, but merely to warn against the anticipation of 
success in all cases simply because of the generahty of Mendehan princi- 
ples. Hybridization, even of varieties, in order to be generally successful 
must be intelhgently performed and in the long run the experimentahst 
who is the most thoroughly informed concerning his plants will stand the 
best chance of securing the improved forms he desires. Each species 

342 



HYBRIDIZATION 343 

has its own morphological and physiological peculiarities and general 
methods will need to be modified to some extent in almost every case. 

General Method. — -Some results of value have come from promis- 
cuous crossing of varieties and species that appeared to give promise of 
desirable combinations. Considerable hybridization has been done in 
this way especially in establishments where large collections are main- 
tained and by seedmen and nurserymen who have undertaken such work 
as a side issue. Some of the most important results of such work have 
been the accidental discovery of unforeseen possibihtics or hmitations in 
crossing. But many important results have come from carefully 
planned and executed experiments and the demands of modern agri- 
culture necessitate systematic procedure in the employment of hyl)ridi- 
zation in plant improvement. Such procedure includes six steps. 

1. Choice of Parents. — This involves two important matters: first, 
decision regarding the object to be attained which imphes thorough 
famiharity with existing conditions affecting crop production; second, 
comparative study of existing varieties or of species that may yield the 
desired result. 

2. Culture of Parent Plants. — Hybridization is painstaking work and 
when carried on extensively it is time-consuming and, therefore, expensive. 
While it is sometimes necessary to use certain plants, especially shrubs 
and trees, wherever they happen to be growing, yet it is always advisable 
to concentrate materials so far as possible and to grow them under protec- 
tion in the breeding garden or greenhouse. Arrangement of the details 
of culture should include consideration of the optimum conditions for 
normal fruitfulness of the intended mother plants. These plants in 
some cases must be kept under observation and prepared for crossing by 
reducing vegetative growth and restricting blooming and the setting of 
fruit. 

3. Protection of Pollen.- — Flowers on intended male parents should be 
guarded in order to prevent contamination with pollen of other plants. 

4. Castration of Hermaphrodite Flowers. — This must be accomplished 
before anthesis and is usually done shortly before the flower opens in 
order to avoid needless mutilation. But in some close-polHnated species 
it is necessary to emasculate very young buds. The operation consists 
of removal of the stamens and can usually be accompUshed easily by 
using a pair of fine pointed forceps or scissors. The castrated flower is 
then protected with some sort of covering until ready for poUination. 
In moncEcious plants it is necessary to guard the young pistillate flowers 
which are to be pollinated. 

5. Pollination. — The transfer of pollen from guarded flowers of the 
male parent to the prepared flowers of the mother plant should be 
accomplished before or just at the time the stigma becomes receptive. 



344 GENETICS IN RELATION TO AGRICULTURE 

In many species this receptive condition of the stigma is evidenced by the 
secretion of a viscid fluid on the stigmatic surface. It has been thought 
that premature polhnation wrought disastrous effects on the resulting 
progeny, but evidence is conflicting on this point. Certain it is that in 
some species, for example, wheat, no untoward results appear from 
pollination at the time of castration. Plants with small, entomophilous 
flowers such as clover and alfalfa may be hybridized by enclosing the 
insects in a cage surrounding the intended mother plant or plants. 

6. Protection of Pollinated Flowers and Developing Seed. — The most 
commonly used device is the paper bag tied with a string or lead wire 
or fastened securely with a copper wire label on which the necessary data 
are written. In many cases ordinary manila bags of suitable size are 
entirely satisfactory. Where wasps give trouble by cutting holes the use 
of bags made of ramie fiber will be found more satisfactory since these 
bags are made with a glossy surface, but even these will give way under 
the attack of wasps in course of time. Bags made of thin paper which 
has been treated with oil or paraffine are best for withstanding insect 
attacks and for use on dehcate plants. Many special devices, such as 
glass or celluloid cylinders plugged with cotton and firmly supported, are 
used upon occasion. 

Method of Hybridizing Maize. — The technique with this plant is 
simple, but when working among plants growing in close proximity to 
each other considerable care is necessary in order to prevent accidental 
crossing. For protection of the pollen manilla or ramie bags, size No. 8, 
are tied over the top of the plant just as the staminate inflorescence 
(tassel) is beginning to appear. The female flowers to be crossed must 
be covered before any of the stigmas (silks) have protruded through the 
tip of the ear and become exposed. The most satisfactory covering for 
this purpose is a strong paper bag about the size of the bags used for the 
tassels. It has been found economical of time to use bags which are 
folded so that the center line of the bottom is exposed (not "square 
bottom" bags) and to slit the bottom, fold over once and fasten with a 
wire clip before covering the ear, with minimum danger of introducing 
foreign pollen. This device makes it possible to examine the develop- 
ment of the stigmas. After stripping off the leaf subtending the young 
ear to be covered the bag is pulled down over the ear as far as possible 
and tied securely to the stem of the plant. When the stigmas are well de- 
veloped and while they are still fresh the bag containing the tassel is 
removed from the intended male parent and carried to the plant which is 
to be polHnated. A hole is torn in one corner of the bag, the top of the bag 
covering the ear to be poUinated is then opened, the pollen is dusted over 
the stigmas and the bag enclosing the ear is closed immediately thereafter 
and securely fastened. Full data concerning the cross are recorded on a 



HYBRIDIZATION 



345 



wooden la])el with copper wires which is attached to the ear. If it is de- 
sired to make a second polUnation the bag containing pollen maybe tied to 
the plant alongside the ear and the same process repeated one or two 
days later. By close observation of the developing stigmas and pollinat- 
ing at the most propitious time well developed ears can be secured from 
a single polhnation. Fig. 142 illustrates the principal features above 
described. 




Fig. 142. — Hybridization of maize. Right, plant just previous to anthesis with leaves 
subtending ears stripped off; left, the same plant with bag enclosing tassel and cylinders 
covering ears. 

Method of Hybridizing Wheat. — ^This plant has numerous hermaph- 
rodite flowers arranged in a branched spike (Fig. 143, upper left). Each 
spikelet bears two rows of bracts or glumes (Fig. 143, 2). The lowest 
two bracts are sterile but each of the next four usually subtends a flower 
while at the top of the spikelet are two or three rudimentary flowers. 
Each flower consists of an ovary with two much branched stigmas (Fig. 
143, 12, 13) and three stamens which are shown in cross-section in Fig. 



346 



GENETICS IN RELATION TO AGRICULTURE 




Fig. 143. — Details of wheat inflorescence. 

The smaller spike is Fife and at its left is shown a Blue Stem spike. In the lower right- 
hand corner is a spike from which small late flowers have been removed preparatory to 
crossing. 

At 2, spikelet, natural size, with a few joints of the rachis;/and g are flowerless glumes; k, 
florets bearing seeds; r, rudimentary florets. 

3, a single flower closed just after flowering, X3. 

4A, longitudinal diagram before flowering, X2.5; anthers marked a; ovary, o; stigma, 
s; filament, /. 

4JB, diagram of floret just after flowering, X3, showing how anthers are held within the 
envelope. 

5, transverse diagrammatic section, or floral plan, as is made by cutting across ^A at x, 
XQ;/g, flowering glume or lemma; p, palea; a, anthers; s, stigma. 

6, flowerless glume; 7, flowering glume or lemma; 8, palea; all slightly reduced. 

9, lodicule, X4, shown also at L in 4B. 

10, cross-section of anther, X26; showing the pollen sacs and the central mass of tissue 
to which they are attached. 



HYBRIDIZATION 



347 



143, 5. The essential organs are completely enclosed by two bracts, 
the floral glume or lemma, which bears an awn in bearded varieties, and 
the smaller palea. The lemma and the palea open for a short time during 







Fig. 144. — Hybridizing wheat. Note position of operator and his equipment, con- 
sisting of a box containing strips of paper and pins for covering the wheat heads and tags 
for labeling, a flask of alcohol for sterilizing the hands and instruments, a pair of forceps and 
a scalpel. Pollinated heads which have been wrapped and labeled are shown at the left. 

anthesis, but as a rule some pollen is shed upon the stigma before the 
flower opens. The flowers remain open only a short time in cool climates 

11, pollen grains, round and smooth, 55 micro-millimeters in diameter. 

12, ovary and stigma just prior to flowering; 13, at the time of flowering; and 14, 
shortly after flowering. 

15, 16 and 17, the mature seed; a, the ventral side; h, the dorsal side; c, the germ or chit; 
s, the stem end of the germs; r, the root end; e, outer layers or bran; d, the incurved surface 
of bran on the ventral side of the seed. The white portions of 16 and 17 are the floury 
interior consisting of cells containing the gluten and starch from which white flour is made. 
(^After Hays and Boss, Minn. A. E. S.) 



348 



GENETICS IN RELATION TO AGRICULTURE 



but a number of natural hybrids in wheat have been reported, especially 
in sub-tropical countries. In the immature flower the anthers are short 
and closely packed around the pistil. Just before anthesis the filaments 
lengthen sufficiently to allow the anthers to protrude when the flower 
opens. Castration can be accomphshed without difficulty by choosing 
flowers nearly ready to open and removing the later flowers on the upper 
portion of the spike as shown in Fig. 143, lower right. With a pair of 
fine forceps the lemma and palea are forced apart and the anthers care- 
fully removed. From a head of the desired male parent anthers just 
ready to burst are then removed and an anther is placed in each of the 
castrated flowers (Fig. 144). Two pairs of forceps should be used, one 




Fig. 145. — Sexual columns of alfalfa flowers (enlarged 7 diameters), showing differ- 
ent stages of development: A and B, anthers just before dehiscence: C and D, anthers 
dehisced; E and F, after treatment with water jet previous to artificial pollination. 
{After Oliver.) 

for castrating and the other for pollinating; or, if the same instrument is 
used, it should be steriHzed by dipping in alcohol between each operation. 
After polhnation the spike is bagged or wrapped with several thicknesses 
of cheesecloth and labeled with a paper string tag. The use of the cloth 
and light weight tag is to be preferred because most grain plants will 
support this extra weight without staking (see Fig. 144). 

A Method of Hybridizing Alfalfa. — An ingenious method of crossing 
this and similar small flowered species has been worked out by Oliver. 
The essential points are as follows : First, have pollen from male parent 
at hand ready to be applied to the prepared stigmas. This is accom- 
plished by taking a flower from a raceme of the male parent, securing the 
banner between the thumb and forefinger and pressing a pin against the 
suture of the keel, beginning at the base and gradually drawing it upward. 
When this is done carefully the stamens and pistil come out gently 
without disturbing the masses of slightly adhesive pollen (see Fig. 145, C, 
D). Now with the aid of self-closing forceps sever the sexual organs from 



HYBRIDIZATION 



349 



the flower and lay aside ready for application to the stigmas of the flowers 
which are to be depollinated. Second, select a raceme in which the 
terminal buds arc about to expand and cut away all the buds and flowers 
except three or four near the center of the raceme. The flowers should 
not be mutilated in any way and should be handled as little as possible. 
In these flowers the stamens will have dehisced perhaps a day or two 
previously but the pollen cannot reach the stigma until the flower is 
tripped. When the tripping is uncontrolled the sexual column (pistil 
and stamens) flies upward and strikes the banner with considerable force 




Fig. 146. — Flowers of alfalfa (enlarged 4 diameters) showing method of depollinating 
and crossing: A, untripped and unpoUinated; B, tripped and self-pollinated; C, tripped 
against a pin to prevent self-pollination and permit depoUination; D, after depollination 
with water jet; E, after artificial pollination; F, after withdrawal of pin the stigma presses 
against the surface of the banner. (After Oliver.) 



and pollen grains are imbedded on the receptive stigmatic surface. It is 
necessary therefore to trip the flower gently and to prevent the stigma 
from touching the banner which is accomplished by inserting a short pin 
between the sexual column and the banner (see Fig. 146, C). Third, 
depollination is accomplished by the use of a fine jet of water from a 
dental chip blower; "the jet may be of sufficient force to remove even the 
empty anthers without injury to the stigma." Then remove the water 
adhering to the flower with a piece of clean, soft blotting paper. Fourth, 
apply the waiting pollen to the depollinated stigma and gently remove 
the pin allowing the stigma to press against the banner (Fig. 146, F). 
''The operation is performed in much less time than it takes to describe 
it and the operator is rewarded by a fairly high percentage of success- 



350 



GENETICS IN RELATION TO AGRICULTURE 



ful crosses." The various implements mentioned above are shown in 
Fig. 147. 

Some of the Difficulties Attending Hybridization. — (a) Different 
Seasons of Maturity. — This is a common obstacle to the crossing of 
different forms. When it involves merely growing periods of unequal 
length the difficulty can be overcome easily by planting at such times 
that the various forms will flower simultaneously. When this is not 
feasible it becomes necessary to resort to some method of preserving the 
pollen. It has been found that pollen of certain species will retain 
vitality for weeks or even months if it is kept very dry. Miss Kellerman 



fefl 


I^^^^^K^^^H ^^H 



Fig. 147. — Implements used in castrating and depollinating hermaphrodite flowers. 
Right, self-closing forceps, ordinary forceps, scissors and scalpel. Left, chip blowers and 
syringes. {Courtesy U. S. Department of Agriculture.) 

reports that the most effective method tried by the Bureau of Plant 
Industry was as follows : anthers were placed in dried vacuum glass tubes, 
e.g., tube filled with anthers 1-2 inches, cotton 3-^ inch, exhausted to about 
0.5 mm. pressure in the presence of sulfuric acid, the tube then sealed. 
As far as practicable the pollen was kept at a temperature of 10°C. A 
simpler and very useful method is to make a double container by fitting 
a small vial inside a larger one and partially filling the space between the 
two with anhydrous calcium chloride or sulfuric acid, filling in the 
upper portion with absorbent cotton and tightly corking the larger 
vial. The anthers or pollen grains are placed in the inner vial after it 
has been thoroughly sterilized and allowed to dry. 

(6) Failure of Fertilization. — This may be due to many causes ranging 
from simple morphological maladjustments to complex physiological 
relations amounting to antagonism. Probably a very frequent cause of 
unsuccessful crosses is failure of the pollen to germinate. When repeated 



HYBRIDIZATION 351 

failures indicate that this may be the difficulty it will l)e worth while to 
try the application of a film of water or weak sugar solution to the surface 
of the stigma before pollination. By the aid of this simple device crosses 
have been secured between certain species of beans which had been 
repeatedly attempted without success. In this connection it may be well 
to give a word of warning. While it is always advisable to ascertain 
what one's predecessors have accomplished or failed to accomplish, the 
hybridizer should remember that both plants and local conditions are 
variable, and what may have been impossible at one place may be possible 
at another. Or the adoption of simple devices such as the water film 
on the stigma may be the determining factor. Much p(;rscverance is 
sometimes necessary. 

(c) Susce-ptihility to Mutilation. — Some plants are much more sensitive 
to mutilation than alfalfa. It appears that some are suceptible to merely 
removing the anthers from the ends of the filaments. In such cases it is 
necessary to resort to special methods for protecting the stigma from 
self-pollination. The details will depend upon the structure of the flower 
and whether it is protandrous or protogynous. 

Conditions favorable for hybridization may be summarized as follows : 
ideal conditions for flowering and fruiting; receptive stigmas; viable 
pollen ; morphological and physiological compatibility between pollen and 
pistil.; resistance of flowers to manipulations. 

Species hybridization is generally more apt to be attended by diffi- 
culties than is the crossing of varieties, although certain varieties of the 
same species have been found mutually incompatible in crossing. In 
general crosses are most successful when made between closely related 
species. The reason for this is clear when the genotypic differences 
between distinct species are considered as differences between homol- 
ogous factors, i.e., factors which condition similar characters as was 
explained in Chapter XII. It is possible that in very closely related 
species the factors conditioning similar morphological and physiological 
characters are themselves similar, if not in a specific sense at least in 
terms of the whole reaction system. The new combinations of these 
similar systems of factors which would be formed in Fi hybrids, would be 
compatible with the vital functioning of the zygote including the produc- 
tion of viable gametes. In widely separated forms, on the other hand, 
the reaction systems must be very different, thus causing corresponding 
reduction in the chances of favorable combinations among the hybrid 
zygotes. While it is impossible to judge with certainty of the possibilities 
of species crosses by somatic resemblances and differences, yet the 
taxonomic relationships of forms it is proposed to hybridize serve as a 
general guide in forming such estimates. No hybrids between different 
plant families are known and few authentic cases of intergeneric crosses 



352 GENETICS IN RELATION TO AGRICULTURE 

have been reported. While many first generation crosses between 
different species are more vigorous than either parent, others are known 
to be exceedingly weak. Unless repeating crosses which have already 
been made, the hybridizer of species is exploring the unknown and there 
is always the possibility that his results may be of interest to science as 
well as of practical value. 

The Svalof Methad of Creating Populations. — Progress in plant im- 
provement by means of hybridization experiments will always be limited 
by the available supply of experts as well as by facilities and time. Any 
method, therefore, that enables the breeder to secure desirable new 
combinations of parental characters without the enormous amount 
of detail involved in a system of pedigree culture, is worthy of serious 
consideration. Such a method was devised by Nilsson-Ehle and has been 
used at Svalof with success. According to Newman, "two known sorts 
are crossed and the whole progeny from all second and succeeding gen- 
erations is sown together en masse. The object of this plan is to allow 
the severe conditions of winter and early spring to either destroy or 
expose the weaknesses of as many of the more delicate combinations as 
possible. In the latter case the breeder is given an opportunity to assist 
nature in her work of elimination by practising a form of mass-selection. 
While there is thus effected in a very simple manner a gradual weeding 
out of a great mass of unfit combinations, the progeny of a crossing at 
the same time gradually assumes the character of an ordinary mixed 
population, the different combnations becoming automatically constant 
as time passes. The advantages of working with constant forms will be 
appreciated by all breeders as will also the fact that through the above 
arrangement the number of combinations which may arise through the 
repeated segregation of inconstant forms in each succeeding generation 
will have increased immensely. . . . While the above system requires 
a considerable length of time before any definite results can be reached, 
yet it requires very little work until the time comes to make selections. 
Numerous crossings of this kind may therefore be carried forward with 
the regular work and thus provide a constant source of new material." 



CHAPTER XX 
UTILIZATION OF HYBRIDS IN PLANT BREEDING 

Although the special uses to which plant hybrids may be put are very 
numerous, they fall into two categories, viz., first, the production of new 
desirable combinations and, second, the production of increased vigor 
in the first hybrid generation. The first category includes all phases of 
the usual purpose of crossing plants, which was briefly discussed in the 
preceding chapter. The new character combinations desired may be 
exclusively morphological or physiological or, as is more often the case, 
they may represent combinations of both kinds of characters involving 
many factors. In the simpler cases involving only a few pairs of inde- 
pendent factors the breeder who is familiar with the Mendelian princi- 
ples of heredity can easily compute the number of Fo individuals that he 
must grow in order to secure the desired combination. Even in the most 
complex cases knowledge of the principles of genetics will be of practical 
value in helping the breeder to understand his results in Fi, F2 and later 
generations and in guiding his selection of F2 individuals for further test- 
ing. These principles are discussed in Chapters V to X. It is the 
purpose of this chapter to present some specific results of the increased 
vigor so commonly observed in Fi hybrids. This increased vigor, or 
heterosis, as it has been termed by Shull, may manifest itself in greater 
size, more rapid growth, larger productivity, greater hardiness, drouth 
resistance, etc. The theoretical explanations of heterosis have been dis- 
cussed in Chapter XII. In the present chapter we shall consider only 
the utilization of the principle that hybridization of closely related 
varieties or species usually results in heterosis. As the methods used 
with plants grown from seed differ from those which can be used with 
vegetatively propagated plants, the two groups will be considered 
separately. 

Increased Production in Fi Maize Hybrids. — This phase of corn 
breeding has come into considerable prominence in recent years. 
Although it has not yet become an important factor in corn growing, 
it presents interesting and important possibilities in the way of increased 
production. The most significant results have been obtained by growing 
Fi hybrids between species, sub-species, commercial varieties, local strains 
of commercial varieties and closely inbred strains or biotypes. The 
earliest recorded experiments on increased production are stated by 
23 353 



354 



GENETICS IN RELATION TO AGRICULTURE 



^, 



Collins to be those of Beal (Michigan, 1878-1882), Ingersoll (Indiana, 
1881), Sanborn (Maine, 1889), and of Morrow and Gardner (Illinois, 
1892). All of these crosses were made between commercial varieties and 
in each case the hybrids outyielded one or both parents. Then came the 
work of Shull and of East (1908) with inbred strains and the crosses between 
them, both investigators obtaining an increase in yield in the hybrids 
over that of the original stock. Following this the United States Depart- 
ment of Agricultm^e conducted experiments on an increasingly extensive 
scale and included work with the most distinct types as well as commejcial 
varieties and inbred strains. More recently various experiment stations 
have conducted similar investigations. 

Crossing inbred strains or hiotypes produces the most striking results 
because the rate of increase in vigor in the Fi hybrids over the inbred 
strains is enormous (as much as 250 per cent, over the average of the 
parents) . Of course it is much greater in some cases than in others be- 
cause of the inherent differences between different bio types. East worked 
with biotypes of four different varieties and secured an average increase 
of 73 per cent, in all crosses. The data on inbreeding the Leaming 
dent variety are summarized by East and Hayes in Table LI. It will be 
noted that two of the strains were not grown as second inbred generations 



Table LI. — Effect of Inbreeding in Strains of Leaming Dent Maize 
Yield in bushels of shelled corn per acre and years in which grown [After East and 

Hayes) 



Parent 


Strain 
number 


Generations inbred 


variety 


1 


2 


3 


4 


5 


6 




6 


59.1 

(1906) 


95.2 

(1908) 


57.9 

(1909) 


80.0 

(1910) 


27.7 
(1911) 




88.0 (1905) 


7 


60.9 
(1906) 


59.3 
(1907) 


■ 46.0 

(1908) 

59.7 
(1909) 


63.2 

(1910) 

68.1 
(1910) 


25.4 

(1911) 

41.3 

(1911) 




9 


42.3 
(1906) 


51.7 

(1908) 


35.4 

(1909) 


47.7 

(1910) 


26.0 
(1911) 






12 


38.1 
(1906) 


32.8 
(1907) 


46.2 
(1908) 


• 23.3 

(1909) 
< 

28.7 
(1909) 


16.5 

(1910) 

9.5 
(1910) 


2.0 

(1911) 

2.0 
(1911) 



UTILIZATION OF HYBRIDS IN PLANT BREEDING 



355 



until 1908 and in that year "the general environmental conditions were 
much above normal. For opposite reasons, poor soil and badly distributed 
rainfall, the yields of 1909 are somewhat too low and the yields of 1911 
are very much too low." With these facts in mind an examination of 
the table shows that the strains became more and more differentiated as 
to yield as inbreeding progressed. "The first strain, No. 6, is a re- 
markably good variety of corn even after five generations of inl)reeding. 
It yielded 80 l)ushels per acre in 1910. ... In the field, even in 1911, 
the plants were uniformly vigorous and healthy and were especially 
remarkable for their low variability. The poorest strain. No. 12, is 
partially sterile, never fills out at the tip of the car and can hardly 




Fig. 148. — Inbred strains of Learning dent corn compared with Fi and F2 hybrid gener- 
ations. The yields per acre were as follows: No. 9 (at the left) 47.7 bu.; No. 12, IG.Gbu.; 
(12 X Q)Fi, 117.5 bu.; (12 X 9) i^s, 91 .5 bu. {After Ea>it and Hayes.) 



exist alone. . . . When two of these inbred strains are again crossed, 
the Fx generation shows an immediate return to normal vigor. The 
plants are earlier and taller, and there is a greater total amount of dry 
matter per plant. For example, in 1911 the average height of all the 
strains of inbred Leaming dent was 84 inches while the average height of 
the 16 hybrid combinations was 111 inches and the height of the shortest 
hybrid combination was 1 foot greater than that of the tallest inbred 
strain." In general it seems that the combinations into which strain 
No. 7 was introduced were the best while those in which the poorest 
strain. No. 12, was used were the poorest. However, a cross between 
these two strains in 1911 yielded 60.2 bushels per acre. The F^ genera- 
tion from a number of the crosses was grown and in every case there was 
a decided falling off in production. This would be expected as a matter 
of course under conditions of random mating in F\ inasmuch as some 
homozygous combinations would be formed among the F^. zygotes. 
Fig. 148 shows types of ears and comparative yields in strain No. 
9 after 4 generations of inbreeding, strain No. 12 in the fifth inbred 
generation, and the Fi and F^ hybrids, all grown in 1910. 



356 GENETICS IN RELATION TO AGRICULTURE 

Theoretically the maintenance of superior near-homozygous strains 
and annual crossing of the best for production of Fi seed for sale to growers 
is a practicable method of corn breeding. This plan was first suggested 
by Shull. It is certainly a desirable method not only because of the high 
degree of heterozygosity produced on crossing such strains, but because 
continuous inbreeding has a similar effect to growing the plants under 
adverse conditions. It tends to eliminate all but the strongest individuals 
and is thus an effective method of selection. However, as a more prac- 
ticable method, East suggested that combinations of the various com- 
mercial varieties be tested until the most profitable combination is 
found. There has been considerable investigation of both methods, but it 
is impossible at present to say which will be used more extensively. One 
of the most valuable features of this method of inbreeding followed by 
crossing of superior strains, as compared with ordinary ear-to-row selection, 
is the saving in time. For example, in the production of high-yielding 
strains of corn which differ in chemical composition of the grains, 
Emerson and East point out that ear-to-row selection from open pol- 
linated plants will, if carried on long enough, produce a strain of the 
desired type. It will be sufficiently homozj^gous to insure comparative 
constancy as regards oil, protein or starch content. At the same time 
a sufficient number of factors for other minor characters will be hetero- 
zygous to insure a fairly vigorous strain. But, on the other hand, 
by self-pollination, together with the same sort of selection, several 
practically homozygous strains of the desired type, high oil for instance, 
"could almost surely have been produced in much less time." These 
strains would doubtless have been unlike for many other characters, so 
that if degree of vigor is dependent upon degree of heterozygosity, the 
crosses between them would doubtless have been abundantly vigorous. 
Or if physiological vigor is conditioned by specific factors, then crosses 
between some of the selected strains vvould doubtless have effected the 
most favorable combinations for maximum vigor. In either case the 
result is the same. "While a few years' time may not be an important 
consideration where the character in question can be determined at 
sight, or by mere weighing or measuring, in breeding work requiring 
costly chemical analysis it is extremely important that the desired re- 
sults be obtained in as few years and, therefore, with as few analyses as 
possible.'"' 

Method of Comparing Yields. — The importance of accuracy and fair- 
ness in comparing the yields of Fi hybrids with their parents has been 
determined by Collins. We give his conclusions verbatim: 

"So large a proportion of first-generation maize hybrids have been found to 
give increased yields and the increase is frequently of such magnitude that the 
utilization of this factor of productiveness becomes a practical question. It is, 



UTILIZATION OF HYBRIDS IN PLANT BREEDING 357 

therefore, highly desii-able to uiulerstand the reasons why some crosses give 
favorable results and others give little or no increase over the yield of the parents. 
A necessary step in this direction is to develop a reliable method of measuring 
the effect of crossing, apart from other factors that influence yield. 

"The development of satisfactory methods of comparing the yield of first- 
generation hybrids with that of their parents has been retarded by (1) a failure 
to fully appreciate the importance of individual diversity in hybrids, (2) the 
abnormal behavior of self-pollinated maize plants, and (3) the difficulty of 
securing for comparison hybrids and parents with identical ancestry. It is 
believed that the method here described avoids these difficulties and affords 
more accurate means of comparing first-generation maize hybrids with their 
parents. 

"The method is illustrated by an experiment in crossing two varieties of 
sweet corn in which it was found that the progenj' from one hybrid ear yielded 
nearly double that of the other hybrid ear involved in the experiment. To have 
taken either ear alone would have led to entirely erroneous conclusions regarding 
the increase secured as a result of crossing. The increase in yield due to crossing 
as measured by the method here proposed was 31 per cent." 

Collins describes his method as follows: 

"To compare the behavior of two varieties, which may be called A and B, 
with that of a hybrid between them, two plants were selected in each variety, 
Ai and Ao in the one variety and B^ and B^ in the other variety. The following 
hand pollinations were made: Ai X A2, A2 X Bi, Bi X B-i, and B2 X Ai. The 
result is two hybrid ears and one cross-pollinated ear of each variety. It is 
believed that the mean yield produced by seed from the two pure seed ears gives 
a fair measure of the effects of hybridization. By making two hybrids involving 
all the plants used in producing the pure seed ears individual differences that 
affect the jdelding power of the pure seed ears are similarly represented in the 
hybrids. Thus, in both the parents and the hybrids the average yield represents 
the mean yielding power of the four parent plants, the only difference being the 
way in which the individuals are combined. 

"To secure the most accurate comparison of the jdeld of the four ears, one 
seed from each of the ears was planted in each hill. The different kinds were 
identified by their relative position in the hill. To place the seeds accurately, 
a board 4 inches square was provided with a small pointed peg 2 inches long at 
each corner. These pegs were forced into the soil at each hill, making four holes, 
one for each of the four kinds, only one seed being p'anted in a hole. 
The board was a'ways placed with two sides of the board parallel to the row. 
It was necessary to exercise extreme care in dropping the seeds to avoid changing 
the position of the kinds. The best way to obviate mistakes of this kind is to 
make all the holes of a row in advance and to go down the row with one kind of 
seed at a time. 

"At harvest time the seed produced by each plant was weighed and recorded 
separately. All hills that lacked one or more plants were excluded and the com- 
parison confined to hills in which all four kinds were represented. The method 
of handling the yields was to determine the mean yield of the four kinds in each 



358 



GENETICS IN RELATION TO AGRICULTURE 



hill and to state the yield of each of the four plants as a percentage of the mean of 
the hill in which it grew. The percentage standing of each kind in all the hills 
was then averaged to secure the final expression of the relative behavior of the 
four kinds. 

"This method of comparison is similar to the ingenious plan originated by 
C. H. Kyle, for use in ear-to-row breeding. Kyle's method is to plant each of 
the ears to be tested in a separate row and in each hill to plant one seed of a stand- 
ard, or check, ear with which all ears are compared. Since comparative and not 
absolute yields are desired in the study of hybrids and with only four kinds to 
compare, the introduction of a check in the present experiment would have 
increased the space occupied by the experiment without lessening the experi- 
mental error." 




Fig. 149. — Parents and Fi hybrid between two sub-species of Zea mays: Hall's Tyler 
dent (left), Brewer's flint (right) and hybrid (center). The hybrid yielded 9 per cent, 
more shelled corn than the dent and 20 per cent, more than the flint and proved the 
most productive of all varieties and crosses in the 1913 test. {After Hayes, Conn. A. E. S.) 



Crossing Species, Sub-species, Varieties and Local Strains.^ — Many 
experiments have been made to test the increase in productivity of F\ 
hybrids between more or less closely related forms of maize. As it is 
impossible to review them all, we give as an illustration Collins' summary 
of the results of 16 crosses made in 1908 between corns of diverse types 
and from widely separated localities. The classification indicated by 
Collins' descriptions are as follows: Zea mays indentata (starchy or dent 
varieties)^ — Maryland, Kansas dent, Brownsville, Chihuahua, Mexican 
dent, Xupha (semi-starch); Zea mays ainylacea (floury variety) — 
Tuscarora; Zea mays everta (pop) — Cinquantino, Algerian, Tom Thumb; 
Zea m.ays indurata (flint) — Guatemala red, Salvador Black; Zea hirta 
Bonafous — Hairy Mexican, Huamamantla, Arribefio; Unclassified — 
Hopi, Chinese (waxy endosperm), Quezaltenango Black, Quarentano. 
The yields of the 16 crosses and of their parents are given in Table 
LII. 



UTIUZATION OF HYBRIDS IN PLANT HREEDlNd 



359 



Table LII. — Yields of 1G Maize Crosses Compahed with 
Parental Yields. {After Collins.) 



Name of liyhrid 



Ahz, Maryland dent by Hopi 

Ah>, Tuscarora by Cinquantino 

Dhi, Kansas dent by Chinese 

Dhi, Chinese by Chihuahua 

Dhz, Hopi by Chinese 

Dhi, Chinese by Xupha 

Brownsville by Chinese 

Hopi by Algerian pop 

Tom Thumb by Quezaltenango black. 

Brownsville by Guatemala red 

Guatemala red by Salvador black . . 

Quarentano by Brownsville 

Huamamantla by Hairy Mexican . . 

Arribeno by Hairy Mexican 

Hairy Mexican by Chinese 

Mexican dent by Tom Thumb 

Average percentage of increase of hy- 
brids over average parents . 



Dh,, 

Eh I, 

Ghi, 

Khn, 

Khi2, 

Mhn, 

Mhu, 

Mhi6, 

Mhn, 

Mho,, 



Yield 

of 
female 
parent, 
pouads 



1.19 
0.53 
0.99 
0.39 
0.74 
0.39 
0.77 
0.74 
0.10 
0.77 
0.31 
0.27 
0.40 
0.39 
0.18 
0.52 



Yield 
of male 
parent, 
pound 



0.74 
0.24 
0.39 
0.69 
0.39 
0.63 
0.39 
0.34 
0.10 
0.31 
0.27 
0.77 
0.18 
0.18 
0.39 
0.10 



Average 
yield of 
parents, 
pound 



0.965 
0.385 
0.690 
0.540 
0.565 
0.510 
0.580 
0.540 
0.100 
0.540 
0.290 
0.520 
0.290 
0.285 
0.285 
0.310 



Yield 

of 

hybrid, 

pounds 



1.25 
0.75 
1.09 
0.95 
1.28 
0.54 
1.16 
0.91 
0.42 
0.49 
0.33 
0.48 
0.31 
0.47 
0.61 
0.54 



PercentaRe of 

increase of 

hybrid over 

average of 

parents, 

per cent. 



29 
95 

58 

76 

126 

6 

100 

69 
(a) 
-9 

14 

-8 

7 

65 
114 

(a) 



53 



(a) Where the yield of either parent fell as low as 0.10 pound per plant the percent- 
age of increase of the hybrid is omitted. In dealing with these small quantities it is 
believed that percentages would be misleading. 



The superior qualities of first-generation hj'brids in maize as set 
forth by ColHns may be summarized as follows: (1) Increased yield. (2) 
Uniformity equal to that of the parents. (3) Quality intermediate 
between parents (but Hayes' data indicate complete dominance of low 
protein over high protein). (4) Increased immunity from disease. 
(5) Extension of the industry into new territory. Especially strong 
evidence for this is found in several of the crosses between diverse tj^pes. 
"Almost without regard to the nature of the parents the hybrids remained 
dark green and vigorous when nearly all the pure strains were giving 
evidence of the lack of moisture by their curved leaves and yellow 
color." (6) Less localisation of highly bred strains. The importance 
of local adjustment in highly bred strains is the chief reason for the 
disappointment which sweet corn growers experience when they purchase 
carefully selected strains from other localities. "First-generation 
hybrids are to a great extent independent of this delicate adjustment to 
local conditions." (7) Increased utilization of the work of experienced 



360 GENETICS IN RELATION TO AGRICULTURE 

breeders. (8) Stimulus to the work of improvement through the possi- 
biUty of protecting new productions. 

More recently Jones and Hayes have made extensive experiments in 
crossing commercial varieties of corn upon which they report as follows: 

"Fifty first generation corn crosses have been compared with their parents. 
Eighty-eight per cent, yielded more than the average and of these 66 per cent, 
yielded more than either parent. 

"In time of ripening the first generation crosses were on the average interme- 
diate when compared with their parents. Thus in crosses between varieties 
differing widely in time of ripening the first generation crosses not only yielded 
more than the late parent but matured considerably earlier. This increase in 
the rate of growth is considered to be fully as important under Connecticut 
conditions as any increase in yield. 

"The highest yielding parents gave the highest yielding crosses as would 
be expected, but a rather unexpected result was obtained in that there was ap- 
parently no relation between the yield of the parents and the increase in the 
yield of the cross. High average yielding parents gave as large increases, when 
stated in per cent., as low yielding parents. 

"There was a tendency for the crosses whose parents differed in their ability 
to yield to give the greatest increase. This is also shown by the fact that the 
dent X flint crosses gave greater increases in growth than the flint x flint crosses. 

"These facts bear out the assumption that hybrid vigor is not the result of 
an indefinite physiological stimulation but merely the result of the bringing to- 
gether of greatest number of favorable growth factors. Crosses between va- 
rieties of diverse type therefore possess a greater total number of favorable 
growth factors than crosses between similar varieties and hence give larger in- 
creases when crossed." 

The immediate effect of crossing upon size of the grain and hence on 
yield should not be confused with the increased production of hybrid 
plants. There is a popular belief that by planting two varieties in 
alternate rows the yield will be increased. That this idea is supported 
by scientific evidence was indicated by the earlier work of Correns, 
Carrier, and Roberts, but it remained for Collins and Kempton to secure 
the proof of this important fact. These investigators used the ingenious 
method of pollinating various white seeded varieties with a mixture of 
their own pollen and pollen from some variety having colored seeds. 
By taking advantage of the phenomenon of xenia they were able to make 
direct comparison of the selfed and the hybrid grains from the same ears. 
The possible invalidity of their results due to more rapid develop- 
ment of hybrid grains and consequent repression of selfed grains was 
removed through the fortunate discovery of an ear that had been twice 
pollinated, first with its own pollen and a week later with pollen from a 
colored variety. "All the white kernels were on the lower portion of 
the ear, all the colored were on the upper portion. Obviously the hybrid 



UTILIZATION OF HYBRIDS IN PLANT BREEDING 361 

seed could have no advantage in this case. The ear produced 212 white, 
or pure, seeds and 161 that were yellow, or hybrid. The average weight 
of the pure seed was 283 gm. per 1000 kernels. The average weight of 
the hybrid seed was 292.5 gm. per 1000, a difference of 9.5 ± 1.06 gm., 
or 3.4 per cent." In the experiment itself eleven ears, involving five 
different varieties, were crossed, giving a total of 1,658 hybrid seeds to be 
compared with 3,513 selfed or pure seeds. In every instance the size of 
the seed was materially increased by the foreign pollen, the increase 
ranging from 2.8 to 21.1 per cent. The fact that the pericarp of the 
mother plant is not strictly a part of the seed but is of purely maternal 
origin might seem difficult to harmonize with the jiesults, but Collins 
and Kempton point out that the necessary increase in size of the pericarp 
would be comparatively slight and to seek any explanation may be 
superfluous. The practical value of this evidence is great. As the 
authors state, "the results afford additional reason for the use of first 
generation hybrid seed; but even where hybrid seed is not to be used, 
the planting of two varieties in alternate rows may be found to increase 
the yields sufficiently to warrant the additional trouble." And further, 
"as the increased size is evidently a manifestation of vigor, it may be 
considered as a factor of adaptation, like the vigor of the first-generation 
hybrid plants. It would seem especially desirable to take advantage of 
this method of increasing yield in regions which do not produce their own 
seed corn." 

Centralized Seed Corn Production. — Carefully selected strains of 
maize are liable to prove disappointing when grown under conditions 
different from those obtaining at the locations where they are produced. 
But the work of intensive selection requires considerable skill and ex- 
perience and the farmer can seldom attend to it properly. He should 
obtain his selected seed corn from a local breeder if possible. The fact 
that 7^1 hybrids in maize are comparatively resistant to local and seasonal 
conditions which prove detrimental to pure strains indicates that such 
hybrids may be produced at central points in quite a large territory. 
When it is known which combination of varieties, or of pure strains of 
a single variety, is best adapted in certain localities, pure seed of these 
varieties or strains may be maintained and the crosses made under 
expert supervision at a central seed farm. On the other hand, a farmer 
who wishes to produce his own hybrid seed need not hesitate on account 
of increased cost of production. Collins has shown that even though the 
cost of raising hybrid seed be double that of ordinary seed, yet "where 
increases ranging from 5 to 50 per cent, may be expected there are few 
farm operations that yield such large returns." 

A Method of Producing Hybrid Corn Seed.— A grower intending 
to produce his own hybrid seed each year might do well by beginning 



362 GENETICS IN RELATION TO AGRICULTURE 

with a series of trials with varieties in alternate rows. After determining 
which varieties are best adapted to the local conditions and give the best 
results when crossed he will be ready to adopt a simple system of hybrid 
seed production somewhat like the following. These directions have been 
sent out by the United States Department of Agriculture to cooperative 
experimenters. Various other plans could be devised. 

Experiments as outlined below involve the use of two varieties and two separate 
plots. Varieties may be designated as No. 1 and No. 2, the plots as A and B. The 
plots should be sufficiently separated to prevent cross-poUination between them. 

It should be kept in mind that the increased yield can be expected only for the one 
year immediately following that in which the cross is made. 

Plot A is planted with alternate rows of No. 1 and No. 2. The rows planted with 
No. 2 are to have all plants detasseled. The crop of No. 1 and No. 2 is to be saved 
separately. 

Plot B is planted entirely with variety No. 2 and has alternate rows detasseled 
The crop from the tasseled and detasseled rows is to be saved separately. 

At harvesting there will be the following lots of seed : 

1. Plot A. Variety No. 1, field-pollinated. 

2. Plot A. Hybrid between No. 1 and No. 2. 

3. Plot B. Variety No. 2, field-pollinated. 

4. Plot B. Variety No. 2, cross-pollinated. 

The yields in the year the cross is made should show the comparative value of the 
two varieties and the effect, if any, of detasseling on the immediate yield. 

A comparison of the yield from these four lots of seed the following year should 
show the yield of the first-generation hybrid as compared with the pure varieties and to 
what extent the increase, if any, is due to the elimination of self -pollinated seed. 

If plot B cannot be provided, seed of variety No. 2 should be held for planting the 
following year in comparison with variety No. 1 and the hybrid seed. 

Application in Other Annual Crop Plants. — The increased vigor due 
to heterozygosis has not yet been utilized in a practical way in annual 
crops other than corn. Melons and other cucurbits are monoecious, 
easily crossed under proper conditions and within a single species, 
and large quantities of seed are produced. Very little is known concerning 
the value of Fi hybrids between varieties as compared with parents, 
but Hayes and Jones report preliminary experiments which indicate 
that first generation cucumber crosses may frequently be expected to 
exceed the higher yielding parent in yield. Only one out of four different 
crosses failed to exceed the average of the parents in any character by an 
appreciable amount. 

In tomato growing Wellington has shown that crossed seed is worth its 
production as based on the increased value of a single crop without ref- 
erence to origin of new varieties. He states that, while desirable results 
have been obtained by crossing plants indiscriminately, "better results 
would undoubtedly have been obtained if high-yielding mothers had been 
selected for one or two generations previous to the first crossing. " Toma- 
toes are normally self -fertilized and, the high-yielding strains or pure Unes 



UTILIZATION OF HYBRIDS IN PLANT BREEDING 3G3 

having been isolated, they can be maintained and the crosses may be 
repeated from time to time. This is a very important consideration for 
the grower who wishes to put the same grade of protkict on the market 
from year to year. "As tomato seed remains fertile from 3 to 7 years, 
a grower does not need to make his crosses oftener than once in 
3 years. The seedsman, as well as the farmer, can profitably raise 
Fi generation seed, provided a guarantee is not given for more than one 
generation, for the buyer, to maintain his quality of product, will have to 
purchase seed every year.'' Welhngton thinks the best results with 
tomatoes can be obtained by keeping within a species and crossing 
distinct varieties or strains. Dominant characters, that will certainly 
appear in the fruits of Fi plants if present in either parent are rough or 
irregular shape, dark red color as contrasted with pink or yellow and 
pink as contrasted with yellow. Size and season of ripening in Fi will 
be intermediate between the parental characters. 

Jones and Hayes report results of similar experiments which corroljo- 
rate Wellington's conclusions. Of two different crosses one (Stone x 
Dwarf Champion) gave an appreciable increase in both size and number 
of fruits and the total yield was thereby increased. It even exceeded 
the better parent by 15 per cent. Moreover, the increase above the latter 
parent was uniform throughout the four years of the test. The other 
cross (Lorillard x Best of All) exceeded slightly the better parent in 
average weight of fruit but it did not excel in total yield. "These re- 
sults show that not all combinations of tomato varieties give the vigor 
usually derived from crossing, but when a desirable combination is found 
it can be counted on to give the increase in yield every time the cross is 
made. Vigor due to crossing as measured by increased yield was not 
appreciably greater in crosses between artificially selfed strains than 
in crosses between ordinary commercial varieties. These results are in 
agreement with the fact that the tomato is naturally almost completely 
self-fertilized. The cross of Stone x Dwarf Champion which gave a 
significant increase in yield also showed a hastening of the time of pro- 
duction. It not only gave a 15 per cent, larger yield than the later 
parental variety but was earlier in its time of production than the earlier 
parent. Hence its value to market gardeners was increased." 

Similarly, in tobacco, Selby and Houser claim that the culture of first- 
generation hybrids will prove both profitable and practicable. Since the 
added cost of producing hybrid seed should not exceed 50 cents per acre 
and the crossing need not be repeated oftener than once in 3 j-ears, 
the financial consideration is neghgible. In regard to uniformity of crop 
they find that Fi hybrids between pure varieties or fixed hybrids show no 
essential difference in uniformity from the parent varieties and for com- 
mercial purposes only such parents should be used. As for yield their 



364 GENETICS IN RELATION TO AGRICULTURE 

results in 1909 showed the average of the hybrids to be about 185 pounds 
more per acre than that of their parents. The maximum increase 
obtained was 492 pounds per acre. By selecting seed from the highest- 
yielding F2 plants it is possible to produce even higher yields in F3 
and Fi. But such high-yielding selections are not fixed and under con- 
ditions of commercial culture the yield and uniformity would undoubt- 
edly decrease rapidly. It appears that the growing of Fi hybrids offers 
the one chance of commercial production of the highest possible yields 
combined with uniformity in size and shape of leaf. The matter of 
quality of cured leaf is more difficult of solution since this is a complex 
character and is easily affected by environmental conditions. Until 
further investigations have been made it seems that the only safe pro- 
cedure is to choose as parents only varieties or strains that produce leaf 
of high quality. 

Application in Vegetatively Propagated Plants. — In this class of plants 
the stimulus due to heterozygosis has been extensively utilized, but this 
has been the result of the method of propagation rather than the conscious 
use of the principle. In potatoes and strawberries, for example, the 
commercial varieties are all hybrids. The crosses having been made, 
the best plant of the first generation became the source of a new variety. 
There are many opportunities for further application of this principle in 
the bush and tree fruits, not only for vigor but for excellence of quahty 
as well; also in asparagus, rhubarb, hemp, hops, pineapples, sugar 
cane, etc. 

It is thought by some horticulturists that the greatest possible im- 
provement in fruits can only be secured by preparing for hybridization 
by several generations of inbreeding. Thus Jones, proceeding on the 
assumption that increase of vigor in hybrids is due to heterozygosis, 
recommends the general adoption of inbreeding in order to secure homo- 
zygous strains which can then be utilized in the production of the most 
vigorous Fi hybrids. But it is to be remembered that only a portion of 
the homozygous strains could be expected to produce superior Fi hy- 
brids. It is, therefore, a serious question whether this method would be 
as economical in the long run as the crossing of existing varieties. 

The use of Fi hybrids as rootstocks for vines, tree fruits and nuts is of 
recognized importance. The Royal and Paradox Walnuts, which were 
named by Burbank from specimens which he produced, furnish a striking 
illustration. The Royal type of hybrid is produced by crossing the 
Black Walnut of eastern states {Juglans nigra) with the California Black 
Walnut (J. calif ornica) ; while the Paradox type comes from crossing the 
walnut of commerce (/. regia) with either of the above named black 
walnuts. Hybrid seedlings commonly appear in the seed beds planted 
with seed from trees standing near trees of other species. As they are 



UTILIZATION OF HYBRIDS IN PLANT BREEDING 365 

easily distinguished by their larger size while still quite small, all that 
the nurseryman has to do is to select the hybrids for budding or grafting. 

Sterility or partial sterility is frequently associated with increased 
vigor in first-generation hybrids between species. The large flowers 
and luxuriant growth of some of the sterile tobacco hybrids render them 
promising subjects for use as ornamentals. Partial sterility, when mani- 
fested by a lessened production of seed, may not be accompanied by 
any decrease in yield of fruits. In such cases therefore it is a positive 
advantage if the plant can b(> propagated vegetatively. 

Rapid-growing timber and ornamental trees of a number of dilfcrent 
species have been produced by crossing distinct forms. Henry men- 
tions the following valuable trees which, on account of their vigor, 
botanical characters and non-occurrence in the wild state, are presumably 
first-generation hybrids: black Italian poplar, London plane, Huntingdon 
elm, cricket-bat willow and the common lime {Tilia vulgaris). Accord- 
ing to Henry the pioneer work on hybridization of trees was done by 
Klotzsch at Berhn in 1845. He crossed two species each of pine, oak, 
elm and alder. He "claimed that by hybridization, both the rapidity 
of growth and the durability of timber of forest trees could be augmented 
considerably; but no further experiments were made, and his pioneer work 
fell into oblivion. " The art of breeding trees was renewed by Burbank's 
work with the walnuts about 1890. Henry reports results with Fi hybrids 
in Populus, Fraxinus, Alnus, Ulmus and Larix. He points out that one 
of his most vigorous hybrids (Popidus generosa) was "derived from two 
parents so little related that they are placed in two distinct sections of the 
genus." At the same time, "a cross between two races of the common 
alder shows considerable vigor, though the parents are so closely allied 
that they can only be distinguished by the most trivial characters." 
Thus it appears that prediction as to the outcome of species crosses in 
trees is quite as impossible as in other classes of plants. There is great 
need for further experimentation. In planting wind-polhnated species 
provision can easily be made for natural hybridization by mixing groups 
of different species. It has been found that the quality of the timber in 
rapid growing Fy hybrids is equal or superior to that of the parents. 

Increased resistance of F\ hybrid plants to insect pests and diseases 
is doubtless often merely another manifestation of their increased vigor. 
But in this connection it is to be remembered that disease resistance is 
generally a heritable character, so that in a particular instance its ap- 
pearance in F\, will depend on the factorial composition of the parents and 
the relation of the factors in inheritance. 



CHAPTER XXI 
MUTATIONS IN PLANT BREEDING 

Discontinuous heritable variations have appeared very frequently in 
cultivated plants under conditions such that they could not be attributed 
to hybridization. The selection of these variations has produced new 
varieties in the same way that the early color varieties of the sweet pea 
arose. By means of breeding experiments many such variations have 
been proved to follow the Mendehan principles of inheritance. The 
general conformity of varietal crosses with the Mendelian principles is 
sufficient reason for asserting that the vast majority of cultivated varieties 
arise either directly or indirectly through factor mutations. A few have 
originated through chromosome aberrations, but, so far as is known, none 
which are of importance to agriculture. It is especially clear that in 
self-fertilized species the production of new varieties from single plant 
selections is made possible by the occurrence of factor mutations. 
The successes of Le Couteur, Shirreff and Hallet, and the achievements of 
Vilmorin, Nilsson, Hays and Johannsen with individual plant selections 
find their explanation in the existence of genetically diverse forms within 
the species or varieties with which they worked. These pure lines must 
have originated through changes in specific factors. Similarly with the 
genotypes of maize and other cross-fertilized species, although new 
combinations of factors are continually arising through natural inter- 
crossing, yet entirely new factors can arise only by means of changes in 
existing factors. These factor mutations do not necessarily induce pro- 
found somatic changes, and slight morphological variations may hardly 
be distinguished from modifications due to environment. When physio- 
logical characters alone are affected, as is sometimes the case, the most 
careful tests of many individuals may be required to discover a desirable 
mutation. But once a mutation arises, the normal range of fluctuation 
in the character or characters affected is different from that of the parent 
form, and new material has been provided for man's selection if he desires 
to use it and can isolate it. When these facts are realized the funda- 
mental importance of mutations to breeding will be appreciated. 

Bud mutations, especially when strikingly different from the parental 
type, have long been known. Bailey states that Carriere in 1856 enu- 
merated over 150 bud-varieties or sports of commercial importance in 
France and he estimated that no fewer than 300 named horticultural 

366 



MUTATIONS IN PLANT liREEDING 307 

varieties grown in this country in 1895 had a similar origin. There is no 
reason to suppose the number has decreased and it is probably larger. 
There is good evidence (see Chapter XIV) to show that bud sports arise 
through factor mutations and that they occur in as great diversity as do 
seed sports. Sometimes striking morphological or substantive changes 
are produced but probably the somatic effect is often slight and hence 
not easily detected (Chapter XXIII). 

Mutations in Crop Plants. — Johannsen has reported two mutations 
in his pure linos of beans. The careful statistical analysis of his successive 
pure line families revealed the first mutant in 1903 and Johannsen thinks 
it appeared as a bud sport. It was characterized by its large size and 
relatively narrow shape. As it was constant from the first it must have 
originated in homozygous condition. The second mutant bore seeds 
which were relatively broad in shape. It could be traced back to 1907 
when it existed in heterozygous condition. Later it was obtained in pure 
line. Very recently mutations of great commercial value occurred in the 
Florida Velvet Bean, Mucuna utilis. The old variety was limited to 
Florida and the Gulf Coast on account of lateness. About 1 ,000,000 acres 
wei'e grown in 1915. We are informed by Piper that early varieties 
originated by mutation at at least three different places, the first in 1906. 
These resulted iii the crop being adapted to the entire cotton belt and 
in a very rapid increase in acreage since 1915. In 1916 about 2,650,000 
acres were grown and in 1917, about 6,000,000 acres. 

Hayes describes a number of mutations in tobacco which is normally 
self-fertilized. The first was found in a homozygous strain of the 
Connecticut Cuban shade variety of commercial tobacco {N. tahacum). 
This strain bears from 14 to 25 leaves per plant, the mean number 
for 1910 and again for 1914 being 19.9 leaves. In 1912 the Windsor 
Tobacco Growers Corporation grew about 100 acres of this strain 
and during the clearing of the field three plants were found that had 
not yet bloomed and which bore a number of uncut leaves. One of 
these was transplanted to the greenhouse of the Connecticut Agri- 
cultural Experiment Station. It produced 72 leaves on the main stem 
and blossomed about January .first. All the seedlings grown from this 
plant came true to the new type which differs from the parent strain 
"in having leaves of a somewhat lighter green shade, in a partial absence 
of basal suckers, and in a practically indeterminate growth " (see Fig. 150) . 
The quality of leaf seems as good as the Cuban and an increased yield 
per acre of approximately 90 per cent, has been obtained, but it is yet 
too soon to know how satisfactorily the new variety will conform to 
trade requirements. Several similar mutants have been found in plan- 
tations of the Connecticut Havana variet5^ This variety has been 
grown in Connecticut for over 50 years and is uniform in habit of growth. 



368 



GENETICS IN RELATION TO AGRICULTURE 



On one of the farms the same mutation has recurred several times. 
Hayes beheves that these mutations cannot be explained as the result 
of accidental crosses. For in the large series of crosses that have been 




Fig. 150. — The Stewart Cuban variety of tobacco, a very promising mutation. Plants 
from seed sown under glass in December and transplanted to the open in May were twelve 
to fourteen feet tall in September and had produced eighty leaves per plant. {From the 
Journal of Heredity.) 

made in the Connecticut station in no case have new forms exhibiting 
this tendency to indeterminate growth been obtained. 

Nilsson-Ehle discovered that in pure lines of oats occasional grains 
appear that are aberrant either in color or in morphological characters. 
The variations tested by him either bred true at once or after one or 
two generations practically all of the progeny would breed true for the 



MUTATIONS IN PLANT BREEDING 369 

new characters. Surface and Zinn consider this sufficient evidence to 
make it "ahnost certain that similarly inherited variations may occur in 
respect to phj^siological characters such as yield." That they were 
justified in making this inference is shown by the success of their ex- 
periments on pure line selections for yield. 

Other self-fertilized crop plants in which mutations have been reported 
are barley, wheat, tomato and potato. 

In maize the sudden appearance of new characters in established vari- 
eties or strains has been reported by a number of investigators. The 
remarkable diversity between inbred strains as discovered by Shull and 
by East indicates the extent to which germinal variations occui' in this 
plant. Each author obtained one strain which was so nearly sterile as to 
be in danger of complete extinction while other strains appeared to be 
capable of maintaining fairly good annual yields indefinitely. Abundant 
evidence of the occurrence of factor mutations in maize is also found in the 
numerous pairs of contrasted characters which are inherited in Mcndelian 
fashion. Besides those referred to in earlier chapters we may mention 
multiple stems (suckers) as dominant over single stems (no suckers), 
normal stature (tall) as dominant (usually) over dwarf stature, normal 
green leaves as dominant to striped leaves, presence and absence of aerial 
roots, hairy and glabrous stems, branched and unbranched tassel, normal 
ear and branched ear, normal anthers and fasciated anthers, presence of 
normal reproductive organs and absence of the same (barrenness). Most 
of these allelomorphs behave as unit characters. Various quantitative 
differences such as stature, ear-length and number of rows to the ear are 
either conditioned by several pairs of factors or by single pairs of factors 
which are subject to a wide range of variability in expression. Con- 
stitutional vigor and productivity are doubtless conditioned by the in- 
teraction of very many factors and a mutation in a single one would alter 
the end result. In short the inherent individuahty of corn plants, which 
makes possible the successful appHcation of selection methods, must be 
referred to factor mutations. 

Sugar beet improvement, particularly increase in sugar content 
of the roots, depends directly upon the occurrence and selection of 
mutations, according to Pritchard. As a result of statistical investiga- 
tions of variation, correlation, inheritance and selection in the sugar 
beet, he concludes that although sugar beet improvement has been ac- 
complished, continuous selection is not necessarily the determinmg 
factor in attaining the present high sugar content of the best varieties. 
His statistics show that the best roots transmit no better qualities 
than do the mediocre roots because the differences are merely "fluctua- 
tions" (modifications). The real differences between sugar beet families 
are usually very slight and are greatly exceeded by their "fluctuations." 

24 



370 GENETICS IN RELATION TO AGRICULTURE 

Both the best and the poorest families transmit average qualities, so that 
continuous selection is not an efficient means of improvement. The 
isolation of mutants, on the other hand, is thought by Pritchard to 
offer promise of improvement, but if the mutation method is to be used, 
it is deemed essential that more efficient experimental methods be 
devised to reduce the effects of soil differences and thus make it possible 
to distinguish real differences more clearly (see Chapter XXV). 

Other normally cross-fertilized crop plants in which mutations are 
known to have occurred are cotton, hemp, rye and the sunflower. 

The Search for Mutations.- — It has long been thought that the two 
most effective methods of inducing heritable variations in plants are 
hybridization and change of environment. Regarding the importance of 
the first there is of course no question, and there is evidence that very 
radical changes of environment such as Tower applied to his beetles and 
White to his tomatoes may induce germinal variation. But the idea that 
mere change of location from warmer to cooler climates or from poorer to 
richer soils, or vice versa, is very effective in " breaking the type" finds very 
little to support it. This notion that culture induces germinal variation 
doubtless finds its explanation in the fact that sooner or later after a plant 
is subjected to intensive culture and close observation new heritable varia- 
tions appear. But why conclude that these variations are induced by 
culture? During the first season of garden cultivation of a species of 
tarweed two mutations were discovered. One was a change in the color of 
the stamens, the other was petallody in the ligulate flowers. It seems 
very probable that these variations would have occurred had the plants 
been growing in the wild. They were found because the plants were 
closely inspected. But is their not fair evidence that cultivation of the 
same species in different regions gives rise to different mutations? There 
is danger of befogging the issue by this question unless we distinguish 
clearly between the origin of mutations and the origin of varieties. To 
consider only one of many possible illustrations : the native sorghums of 
South Africa, the Sudan, Egypt, Arabia and Persia, India, and Manchuria 
are a diverse lot of forms; yet sorghum undoubtedly originated in 
Africa and spread thence to the various regions where it now exists as 
distinct varieties. It must be admitted that different varieties have 
developed in different regions, but does this necessarily indicate that geo- 
graphical differences actually caused the original germinal alterations 
which resulted in the different varieties of sorghum? Such a conclusion 
seems unwarranted in view of what is actually known concerning the 
occurrence of mutations under both natural and artificial conditions. 
Moreover, it is not improbable that the progenitors of existing varieties 
of sorghum all originated in Africa, although geographical differences may 
have been the determining factor in the survival of those mutations 



MUTATIONS IN PLANT BREEDING 371 

which gave rise to existing varieties. Factor mutations conform in 
their manner of occurrence with de Vries' mutation theory; they arise 
suddenly, they occur in all directions, they are heritable, and some of 
them are advantageous to the species and are preserved by natural 
selection. When so preserved they give rise to new forms or races, and 
when fostered by man they make possible new horticultural varieties of 
l)lants or new breeds of animals. But as yet we have no ground for 
asserting that factor mutations are caused by geographical differences 
or by any specific elements of the environment. From his study of varia- 
tion in tobacco Hayes reached the conclusion that while environment is 
of great importance in breeding tobacco as well as in growing the commer- 
cial product, yet change of environment "does not cause a breaking up of 
type, and whatever variations occur due to environment appear alike in all 
plants of a particular type." Thus it appears that mutations arise quite 
independently of conditions of culture, and it is probable that they ai-e 
somewhat more frequent than has generally been supposed. It is certain 
that mutations which are undesirable for agricultural purposes are quite 
as apt to occur as are desirable ones. For this reason neglect of seed selec- 
tion has caused the "running out" of many good varieties. The breeder 
who would improve the best existing varieties which are adapted to a 
given location must either resort to hybridization between the varieties or 
else search for the most desirable biotypes within each variety. Even 
though hybridization is clearly necessary from the first, it may well be 
preceded by a systematic search for the best forms within the varieties to 
be crossed. 

One of the most successful attempts to take advantage of the desirable 
mutations which had occurred within cultivated varieties was carried out 
by Surface and Zinn at the Maine Experiment Station in their experi- 
ments on oat breeding. Oats being self-fertilized, they assumed that 
any new characters which had originated would breed true. The two 
most desirable improvements in the commercial varieties of oats culti- 
vated in Maine are increase in yield and in strength of straw. Accord- 
ingly individual oat plants were selected with these two points in mind. 
This work began in 1910 when 460 plants were chosen from 18 different 
commercial varieties. Of these only 188 were selected for planting in 
1911 and on the basis of the results obtained 80 were continued for test- 
ing in duplicate 1-2000 acre plots in 1912. Of these pure lines 34 were 
sufficiently promising to be continued into field tests in 1913. Thirty- 
one of these were again tested in 1914. In 1915 all of these pure lines 
were discarded except 12 and these were tested in quadruplicate plots in 
1915. In each of the 3 years 1913-15 these pure lines were grown along 
with a number of the best commercial varieties obtainable. In 1914-15 
the pure line plots alternated in the field with commercial variety plots. 



372 GENETICS IN RELATION TO AGRICULTURE 

After correcting the yield of individual plots for differences in soil fer- 
tility (see Chapter XXV) it was found that the 12 pure lines averaged 
to yield 80.8 bushels per acre against 75.2 bushels for the 11 commercial 
varieties. "Only 4 of the commercial varieties gave a better yield 
than the poorest of the pure lines. In all cases the average yield of the 
pure lines selected from a given variety exceeded the yield of the parent 
variety." Of the 18 commercial varieties with which they started only 
3 are represented among the 12 pure lines. It was found that these pure 
lines closely resemble their respective parents in morphological characters 
and concluded, therefore, that mutations in the physiological characters 
which result in higher yield are not necessarily associated with changes 
in morphological characters of the plant or grain. 

Even more striking results in some respects have been obtained by 
Clark by means of head-to-row selections of Ghirka spring wheat in North 
Dakota. Starting with 300 individual plant selections in 1909, in spite 
of the destruction of all the cultures by hail in 1912, after 5 years' 
work two pure lines were found, one of which was superior to unselected 
Ghirka in all characters except crude protein content and the other in all 
characters except volume of the baked loaf. 

In all such work the importance of beginning with large numbers must 
be emphasized. Other things being equal as the number of selected in- 
dividuals increases the chances of locating the desired variants increase. 
The same holds true when attempting to locate aberrant individuals by 
inspection of young seedlings, nursery stock, etc. 

Propagation of Mutations. — The question of how to preserve and prop- 
pagate a desirable mutation becomes a problem only in plants which are 
normally cross-fertilized. In self-fertilized species the new form exists 
as a pure line and need only be isolated. Similarly in plants that are 
propagated vegetatively usually there is no difficulty in multiplying a 
new variet3^ This method has been applied to such crop plants as 
alfalfa with great success. But in a cross-fertilized plant in which 
vegetative propagation is impracticable the method of procedure will 
depend upon circumstances. If in a given species the plants are self- 
fertile the appearance of only a single plant of a new form makes it possible 
to test its genetic constitution and if it is a mutation to multiply it. If 
it is heterozygous for the factor conditioning the new character or char- 
acters it will of course be necessary to select the best individuals from 
the next generation. This situation will confront the breeder only in the 
case of mutant factors which are dominant or partially dominant when in 
the heterozygous condition. When the mutant factor shows partial 
dominance in a heterozygote but segregates as a Mendelian recessive 
there will be no difficulty in establishing a pure strain, but should it 
segregate as a Mendelian dominant it becomes necessary to test a number 



MUTATIONS IN PLANT BREEDING 373 

of the seedlings exhibiting the new characters. In cross-fertiHzcd species, 
in which individual plants are self-sterile, where a mutation appears 
in only one plant, several successive crosses may be necessary in order 
to produce a strain which breeds true for the new type. It must be 
crossed back on the parental form to begin with. If the change from 
the parental type is conditioned by a single factor the number of hybrid 
generations to be raised will depend on whether that factor segregates as a 
dominant or a recessive. In the latter case a true breeding strain should 
be obtained in the second generation but in the former it will require 
three or more hybrid generations depending on the extent to which the 
new characters depend upon environmental conditions for their expression. 



CHAPTER XXII 
GRAFT-HYBRIDS AND OTHER CHIMERAS 

A graft-hybrid, as its name implies, is a shoot or plant which is pro- 
duced by grafting one kind of plant upon another and whose characters 
are intermediate between the characters of the two components. None 
of the so-called graft-hybrids are really hybrids at all; they are merely 
mixtures of tissues from two kinds of plants which can live in unison. But 
each kind of tissue is distinct in its every cell, i.e., there has been no fusion 
of cells or blending of germ plasm as in the case of sexually produced 
hybrids. Some difference of opinion still exists regarding a single case 
which will be referred to again, but the above statement certainly applies 
to all other graft-hybrids that have been investigated. As Buder says, 
a graft-hybrid is nothing else than a special form of graft-symbiosis. 
Thus all graft-hj'brids are chimeras (p. 271). A conception of how chi- 
meras originate naturally may be gained by learning how graft-hybrids 
have been produced experimentally. 

Tomato -nightshade Graft-hybrids. — Some members of the night- 
shade family are easily grafted even though they belong in different 
genera. Thus it is possible to double work nightshade on tobacco on 
tomato. Reciprocal grafts of tomato and potato are easily made and 
Heuer has grafted tomato on egg plant and tomato on bittersweet. 
In recent years Winkler has produced four different chimeras and another 
form which he considers a true hybrid by grafting tomato on nightshade 
or vice versa. The four Solanum chimeras are shown in Fig. 151. Wink- 
ler's method is to graft on a scion by one of the ordinary methods and 
soon after it has united with the stock to cut it off taking pains to make 
the cut pass through the united tissues of stock and scion as shown in 
Fig. 152. Most of the adventitious buds pushed out are either night- 
shade or tomato. But occasionally a bud will be formed on or near the 
line of union. In such cases either one of two combinations of the graft- 
components may result depending on the relations of the two kinds of 
callous tissue. If the two masses meet as in a of Fig. 153, the young 
shoot will consist of sectors of nightshade and tomato, but should one of 
the cell-masses grow over the other tissue producing the condition shown 
in 6, the young shoot in this case will be composed entirely of tomato 
inside but will have an envelop of nightshade cells surrounding it. The 

374 



GRAFT-in URJlJ.S A.\JJ OTIIKk CHIMERAS 



375 



first type is termed sectorial and the second periclinal. A third type 
has been recognized \)y Winkler in which the vegetative cone is a mosaic 
of unlike cells. This type he named hyperchimera. Coit has reported 
the case of a Valencia orange tree whi(;h from its consistent instability 
appears to have been propagated from a mixed bud and hence belongs in 
this class of chimeras. 

All of Winkler's Solanum chimeras are periclinal and the degree of 
lesemblance of such a graft-symbiont to the parent whose tissue comprises 
the inner portion of the shoot seems to depend upon the number of layers 
of cells from the other parent which envelop it. Thus the form luhujense 




A li 

Fig. 151. — Winkler's Solanum chimeras. Produced by grafting tomato on nightshade 
(and vice versa). From left to right, Solanum Gaertnerianum, S. Koelreuterianum, S. 
proteus, and S. luhigense. The second resembles the tomato parent most closely; it has a 
tomato body with nightshade epidermis (a single layer of cells;. The fourth is most like the 
nightshade parent and it has a nightshade body with tomato epidermis. The first has a 
tomato body covered with 2 layers of nightshade cells and the third, a nightshade }>ody 
covered with two layers of tomato cells. Such combinations are called periclinal chimeras. 
{From Journal of Heredity.) 



(Fig. 151, IJ) wliich closely resembles the nightshade has all the inner 
portion of ;S. nigrum, with just an epidermal layer, one cell thick, of S. 
lycfjper.sicum. But proteus (Fig. 151, C), whose leaves are much more 
like tomato leaves, has a double layer of tomato cells overlying the 
nightshade body. Similarly, Koelreuterianum (Fig. 151, B) is really a 
tomato with nightshade epidermis, while Gcertnerianum has a tomato 
body covered with two layers of nightshade cells. These graft-hj'brids 
were discovered only after much patient work in the course of which 
Winkler made 208 grafts which produced more than 3000 shoots. All 
four were propagated from cuttings and by this method they have been 
obtained and grown by the New York Botanical Garden. 

It has not been possible to compare these forms with true sexual 
hybrids because no one has yet succeeded in crossing the tomato and the 



376 



GENETICS IN RELATION TO AGRICULTURE 



nightshade. But sufficient evidence of their mixed composition is found 
in the character of their progeny and from the study of chromosome 
numbers made by Winkler. In tuhigense the fruits are almost identical 




Fig. 152. — Diagrams showing methods of grafting used in producing the tomato- 
nightshade chimeras and some of the results; shaded portions represent scion tissue, un- 
shaded, stock tissue, a, Splice graft; b, cleft or wedge graft; c, saddle graft; d, sectorial 
chimera (shaded portion, nightshade; unshaded portion, tomato tissue); e, chimera leaf, 
part nightshade, part tomato; /, nightshade; g, periclinal chimera, Solanum. tuhigense; 
h, Tomato. {After Winkler from White.) 

with those of the nightshade and the seeds produce only pure nightshade 
plants. The reason for this is clear when it is remembered that this 
form is really nightshade with a single epidermal layer of tomato cells, since 



GRAFT-HYBRIDS AND OTHER CHIMERAS 377 

the germ cells arise from the sub-epidermal layer. The seedlings of 
Gcertnerianum are also pure nightshade because, although this form 
consists of tomato tissue within, it is enveloped by two layers of night- 
shade cells. Similarly with proteiis, which is a nightshade except for its 
two outer layers of tomato cells, the fruits resemble tomatoes and from 
the seed pure tomatoes have been raised. The other form, Koslreuterianum, 
fails to produce fruits. The chromosome numbers are 24 for the tomato 
and 72 for the nightshade. If a fusion of nuclei involving diploid numbers 
had occurred the cells of the supposed hybrids should contain 96 chromo- 
somes, but the only counts obtained by Winkler in the four chimeras 
were 24 and 72. Thus it appears that in each graft-symbiont the two 
kinds of tissue maintain their identity. Yet there is a combined effect 
on the morphological characters. The physiological interactions too 
are such as to cause reduced vigor. This effect is least noticeable in 
Kcelreuterianum which is sterile. The fifth new form, which Winkler 




Fig. 153. — Diagram to show formation of adventitious buds arising at the point of 
union of the two graft-components A and B. a, represents a sectorial combination; b, a 
periclinal combination. (After Buder.) 

claims is a true graft-hybrid, was named Darwinianuni. It appeared on 
one of the shoots from a decapitated graft. Winkler claims the chromo- 
some number of this form is 48 and that certain if not all of the tissues of 
this plant are composed of cells derived from the actual fusion of tomato 
with nightshade cells which involved nuclear fusion. If this is actually 
the case S. Darwinianum is a hybrid in the strict sense and the only one 
known to have been produced by vegetative means. However, Baur 
points out that Winkler bases his claim for this number on the ground 
that he found 24 chromosomes in the pollen mother-cells which arise 
from the sub-epidermal layer. Baur thinks that Winkler's interpretation 
is unwarranted. He believes it much more probable that S. Darwin- 
ianum is a periclinal chimera with a nightshade epidermis, then a sub- 
epidermal layer of tomato cells and the adjoining inner tissues of night- 
shade. Then the number found, 24, is the diploid number of the tomato 
and the reduction division, according to this explanation, is omitted in 
the tomato pollen mother-cells which, in this chimera, are bounded on 
both sides by nightshade tissues; or else it occurs at an unusually late 
stage in development. ''I cannot admit," says Baur, "that the exist- 
ence of real graft-hybrids in the strictest sense of the word is proven." 



378 



GENETICS IN RELATION TO AGRICULTURE 



Since the true nature of Winkler's chimeras has been made clear a 
number of historical cases of graft-hybrids have been investigated. The 
results of this work have been summarized by Buder whose list appears 
in Table LIII. Most of these cases have been fully discussed in other 
works. Typical leaves of the two types of Cratsegomespilus and of 
the two parents, the whitethorn and the medlar, are illustrated in 
Fig. 154. 

Table LIII. — List of the Most Important Periclinal Chimeras Produced 
BY Grafting. {Adapted from Buder.) 



Name and origin 



Used in grafting 



As stock 



As scion 



Composition 



tiahurnumiCytisus) Adami , arose 
spontaneously in 1826 from an 
unsuccessful graft. 

The Cratsegomespili of Bron- 
vaux, originated spontaneous- 
ly many decades ago at Bron- 
vaux in Metz from places 
where stock and scion had 
overgrown on grafts nearly a 
century old. 

(a) Cr. Asnieresi (resembling 
whitethorn) 

(b) Cr. Dardari (resembling 
medlar) 

The Cratsegomespili of La- 
grange, apparently complete 
analogues of the two forms 
from Bronvaux. 

The pear-quince "hybrid" of 
Fr^re Henri, originated about 
1903 in Rennes. 

The peach-almond graft hybrid 
of Daniel and Delpon, arose 
spontaneously in 1908 at Mas- 
Grenier (Tarn and Garonne).' 

The tomato-nightshade "hy- 
brids" of Winkler, produced 
experimentally in 1907-9. 

(rt) S. tubigense 

(b) S. proteus 



Laburnum vulgare 
(Shower of Gold). 



Cratmgus monogy- 
na (Whitethorn). 



Cydonia (Quince). 



Amygdaius com- 
rminis (Almond). 



Solanum lycoper- 
sicum (Tomato). 



Cytisus purpureus 
(Purple Broom). 

Mespilus german- 
ica (Medlar). 



Pyrus (Pear). 



Amygdaius persica 
(Peach). 



Solanum nigrum 
(Black nightshade) 



(c) S. Kcelreuteriatium. 

(d) S. Gwrtnerianum. . . 

(e) S. Darwinianum. . . 



The tomato-eggplant and to- 
mato-bittersweet "hybrids" of 
Heuer, produced experiment- 
ally in 1910. 
Form I 



Form II. 



The populus "hybrid" of Baur, 
produced experimentally in 
1911. 



Solanum, lycoper- 
sicum (Tomato). 

Solannm lycoper- 
sicum (Tomato) 

Populus canaden- 



Solanum melongena 

(Egg plant). 
S. dtdcamara 

(Bittersweet). 
P. trichocarpa. 



According to Buder only one 
outer layer of C. purpureus, 
all within being L. vulgare. 
According to Baur and H. 
Mayer both forms have a 
Cratsegus body which is cov- 
ered by a Mespilus mantel: 



In (a) of one layer of cells. 
In (6) of two layers of cells. 



Probably consists of pear tis- 
sue within a layer of quince 
cells. 

Evidently a mixture of sec- 
torial and periclinal chim- 



According to Winkler: 

Outer layers Inner tissue 
1 of S. lycopers. S. nigrum 
1 and 2 S. lyco- S. nigrum 

persicum 
1 of S. nigrum S. lycopers. 
1 and 2 S. nigrum S. lycopers. 
".\n actual hybrid" (but see 

text). 



Probably only the epiderrnis 
is egg plant, tomato within. 

Epidermis of tomato, inner 
portion bittersweet. 

Only the epidermis of P. tri- 
chocarpa, within P. canaden- 



In addition to the above there are the Bizarria as they have been termed. These are periclinal 
chimeras (some of them perhaps also sectorial chimeras) between different species of Citrus: Pomeranze, 
Citrone, Cedrate, Limette. The earliest record of these dates from Florence, 1644. They aroused in- 
terest in their day because of the manifold sectorial and periclinal chimera combinations in their fruits. 
Although most of these forms are now forgotten, several are still in cultivation, but they have not 
received close study (but see Coit, "Citrus Fruits"). 

Baur's Investigation of a Natural Chimera. — The key to the ex- 
planation of Winkler's artificially produced chimeras was furnished by 
Baur's discovery of the difference between the white-edged and solid 



GRAFT-llYHRIDS AND OTIIEU CIIIMERAH 



:37i) 



green vajieties of geranium {Pelargonium zonale). From his stud}- of 
seedlings of the white-edged variety he had come to reahze that the 
color of the leaves on a seedling depends entirely upon the nature of the 
cells composing the vegetative cone or plumule. This led him to examine 




a b c d 

Fig. 154. — Leaves of the CratjEgomespili of Broiivaux and their components. ' a, 
Mespilus germanica (Medlar); d, Crataegus monogyna (Whitethorn); b, Cratoegoniespilus 
Dardari, with two outer layers of medlar cells; c, Cratwgomespilus Asnieresi, with one outer 
layer of medlar cells. (After Buder.) 

the cells in white-edged and green leaves and he found that in a white-edged 
leaf there is an extra layer of colorless cells in addition to the true epidermis 
(see Figs. 155, 156 and 157). He concluded that a plant bearing all white- 
edged leaves must have a complete peripheral layer of the colorless cells 
just below the epidermis as shown in Fig. 157, a, and that a plant differing 




Fig. 155.— Leaves from periclinal chimeras of the white-edged geranium ; a, from a plant 
with two white peripheral cell-layers; b, from a plant with only one epidermal layer of 
colorless cells. {After Baur.) 

in this respect from a normal green plant should be considered a peri- 
clinal chimera. He had observed sectorial chimeras among his geranium 
seedlings and found that occasionally a plant having some of its leaves 
entirely green and some of them entirely white would produce a shoot 
bearing white-edged leaves. He found that such shoots arose near the 



380 



GENETICS IN RELATION TO AGRICULTURE 




Fig. 156. — Sections through edges of the two leaves shown in Fig. 155. Green tissue 
indicated by stippling. For much enlarged views of the portions enclosed by the small 
rectangles see Fig. 157. (After Baur.) 




Fig. 157. — Microscopical views of those portions of the cross-sections in Fig. 156 
represented by the small rectangles. Colorless chromatophores are indicated in outline, 
green in black. {After Baur.) 





A B 

Fig. 158. — A, Diagram of longitudinal section through a young shoot of a plant bearing 
white-edged leaves — a periclinal chimera. B, Cross-section of stem of a sectorial chimera. 
A bud pushing out at a would produce a sectorial chimera, while one arising at h would form 
a periclinal chimera. {After Baur.) 



GRAFT-HYBRIDS AND OTHER CHIMERAS 381 

l)ouiitlaiy line between the green and the white tissue of the stem. Hence 
he conchided that in order to have a perichnal chimera arise from a 
sectorial chimera the relation between the two kinds of tissue would have 
to be as shown at b in Fig. 158. A bud pushing out at such a point would 
have an envelope of colorless cells in addition to the colorless epidermis. 

Without doubt this structural principle explains the origin of all 
natural chimeras. But this principle holds only when there are groups 
of normally homogeneous tissues in the same stem or bud which have come 
to differ with respect to one or more characters. In graft-hybrids such 
diverse tissues come from different plants. The question of the natural 
origin within the same plant of morphological and physiological differ- 
ences causing somatic heterogeneity where homogeneity is the ordinary 
condition is a problem of far greater fundamental importance. It has 
already been shown that probably all such diversities within single 
individuals arise as factor mutations (Chap. XIV). 

Other Natural Chimeras. — Sectorial chimeras caused by mutations 
in color factors are the most common natural chimeras. They occur 
very frequently in citrous fruits, especially in the orange and lemon 
(see Fig. 161). Other chimeras in these fruits are caused by factor dif- 
ferences affecting thickness and texture of rind and frequently associated 
with these are differences in color and flavor of the pulp. Color chimeras 
are also fairly common in apples and pears and they have been found in 
grapes, olives and tomatoes, as well as in gladiolus, poppies, sunflowers, 
dahlias, and doubtless many other flowers. Many valuable variegated 
forms of ornamental shrubs are mixtures of sectorial and periclinal chi- 
meras with normal green vegetative parts. Some striking examples are the 
Variegated Black Elderberry {Samhucus nigra variegata), the Variegated 
Deeringia {Deerimjia celosioides) and the variegated forms of the Japanese 
Spindle Tree or Strawberry Bush {Euonymus japonicus) . Variegated 
foliage which is caused by factor mutations causing complete or partial 
chlorophyll reduction are also fairly common among herbaceous plants. 

Two Categories of Variegation. — The variegated plants mentioned 
above, like the white-edged geranium, can be propagated asexually and 
it is known that in the geranium, snapdragon, four-o'-clock, maize and 
other plants the variegated character can be transmitted to sexually 
produced offspring. However, certain variegated plants cannot transmit 
variegation through the seed although it is transmissible by means of 
vegetative propagation. Baur has shown that in the latter class, 
variegation results from a pathological condition and by double working 
susceptible and immune stocks he determined that it must be caused by 
a toxin produced by the diseased cells. The long familiar cases of 
"graft infection" among the Malvaceae are thus explained. It seems 
that all cases of "infectious chlorosis" in this family can be traced back 



382 GENETICS IN RELATION TO AGRICULTURE 

to a single variegated specimen of Ahutilon striatum Dicks., which was 
introduced into Europe in 1868 and named Ahutilon thomsoni. Miss 
Reid has shown, however, that among the flowering maples (Abutilon) 
the variegated forms can be grouped into two classes: "those with a 
mottled variegation which is infectious and those with a non-infectious 
variegation with the white cells at the periphery. Both types are of 
importance in horticulture, especially for use as bedding plants; both 
types are of special scientific interest." 

The Physiological Behavior of Graft-hybrids. — Although chromosome 
counts and progeny tests indicate that the cells of each graft-symbiont 
maintain their identity independently of the close proximity of foreign 
cells, yet the intermediate characters of graft-hybrids indicate that the 
components have a mutual influence upon each other. This influence 
is especially notable in the manifestation of physiological activity in- 
volving the whole plant. None of the Solanum graft-hybrids are as 
vigorous as either component under normal conditions. In fact, it is 
with considerable difficulty that they are maintained by means of cut- 
tings, except in the case of Koelreuterianum. This lack of vegetative 
vigor may not be characteristic of all graft-hybrids but it seems to be 
common to most of them. Lack of vigor exists in many natural chimeras 
also, especially in those involving chlorophyll reduction. 

In the Solanum graft-hybrids the germ cells of the two components 
are not equally susceptible to the effect of adjacent foreign cells. In both 
tuhigense and Gaertnerianum the fruits contain fertile seeds which pro- 
duce only pure nightshade plants. But in proteus only part of the seeds 
are viable and these produce tomato seedlings, while in Koelreuterianum 
the flowers are entirely sterile. Similarly, in Cytisus Adami the "hybrid " 
(intermediate) flowers are sterile. 

Graft-hybrids offer many interesting possibilities as a means of studying 
the physiology of development as influenced by the reciprocal relations 
between the components. This method of attack has been utilized much 
more extensively in the study of development in animals than in plants. 
Crampton, for example, grafted together the pupae of different species 
of moths thus producing double monsters. From the specific effects 
upon pigmentation iji some of the graft symbionts it was concluded that 
the pigmental colors in some species are derived from the haemolymph 
by processes of drying and decomposition which are regulated by some 
specific internal factor. Crampton mentions several other investigators 
who performed similar experiments on animals. Buder has suggested the re- 
ciprocal grafting of male and female plants in dioecious species as a means of 
investigating the physiology of sex determination in plants. This suggests 
the whole field of reciprocal effects between scion and stock, concerning 
which there has been considerable investigation in recent decades. 



GRAFT-HYBRIDS AND OTHER CAIMERAS 383 

Modification of One Graft-symbiont by the Other. — The repressing 
or stimulating effects of certain scions on certain stocks is well known. 
Excellent examples are found in the various "dwarf" rootstocks used in 
the culture of the pome and citrous fruits. Besides the dwarfing effect 
of the stock upon the scion, there is often a reciprocal stimulating effect 
of the scion upon the stock which causes the latter to increase in diameter 
faster than the scion. A mutually stimulating effect is sometimes ob- 
served, as in the almond and the peach when used as graft components. 
The importance of selecting stock of about the same vigor, when grafted, 
as the scion has long been recognized by nurserymen. The fact that 
grafts usually exhibit a certain amount of modification according to the 
kind of stock used has given rise to many reputed cases of deleterious 
effects and extreme modifications due to grafting. 

A matter of considerable economic importance involving this question 
concerns the culture of wine grapes. After the introduction of American 
vines and their hybrids into the phylloxera-infested districts of France 
there was widespread concern over the possibility that the quality of the 
French wines would be injured by grafting on the new stocks. Many 
investigations were carried on. Although in the earlier stages of the 
work some very definite effects of stock on scion were reported, the 
evidence as a whole is considered by leading investigators as indicating 
merely that the stock may either increase or decrease the capacity of the 
scion according to the combination used. It has been concluded that 
where due account is taken of affinity of stock and scion, if other condi- 
tions are favorable, grafting has caused no deleterious effect on yield or 
quality. 

A specific, case of supposed deleterious effects attributed to the in- 
fluence of stock on scion was observed by Paelinck. A dark-red variety 
of cherry. Early Rivers, was grafted on mahaleb stock. The resulting 
tree bore fruit which was yellowish white in color, of smaller size and 
which matured 8 days later than Early Rivers. Scions from this white- 
fruited tree were grafted on mazzard stock which has small black fruit 
"to see whether the white fruits would revert to the dark color." The 
result as one would expect was negative. Undoubtedly this was a case 
of bud mutation. 

Other reputed extreme effects of graft-symbionts involve the supposed 
transfer of characters from the one to the other. Baur asserts that after 
reviewing the accounts of many grafting experiments he has reached 
the conclusion that most of the reciprocal effects between stock and 
scion can be explained on the basis of modification in nourishment. 
Moreover, where this explanation does not hold there is a more probable 
cause than the notion of transfer of characters. For example, in the case 
of Daniel's eggplant which, when grafted on tomato bore tomato-shaped 



384 GENETICS IN RELATION TO AGRICULTURE 

fruits, Baur states that there are varieties of eggplant which occasionally 
bear tomato-shaped fruits even when not grafted, and that Daniel 
probably used such a variety. Again Daniel and Elder have reported 
experiments tending to show that the seedlings of scions exhibited an 
influence of the stock. Baur is inclined to think that accidental cross- 
pollination must explain such cases. But Daniel has recently reported 
similar results when working with different varieties of beans. In this 
case, however, there is the possibility that the seedlings of the scions 
and the seedlings used for comparison, which were from ungrafted plants, 
belonged to different pure lines. Thus in some such simple manner all the 
supposed cases of transference of morphological characters may be 
explained. 

Regarding the actual transference of the chemical constituents of the 
tissues from stock to scion and vice versa, the results of experiments 
differ with respect to different plant ingredients. Thus, according to 
Guignard, glucosids do not pass from one graft component to the other 
when the two contain different kinds of glucosids, and the glucosids 
present in plants are apt to differ unless the plants are closely related. In 
graft-symbionts whose components belong to different species, Guignard 
thinks that each component tends to retain its own chemical properties. 
On the other hand, Meyer and Schmidt found that alkaloids such as 
nicotine will pass from a tobacco scion into a potato stock. This is a 
promising field for future investigation. 



CHAPTER XXIII 
BUD SELECTION 

The efficacy and practicability of bud selection is a subject of con- 
siderable interest especially among horticulturists. During the past de- 
cade it has received more and more attention from investigators until now 
there are under way a number of comprehensive projects which, in future 
years, should furnish definite information concerning the more important 
vegetatively propagated crop plants. If it is determined that bud selec- 
tion is an effective method of improving certain varieties either by secur- 
ing increased yield or by the discovery of superior strains, its importance 
to horticulture will have been demonstrated. It will still remain for 
horticulturists to decide as to the practicability of introducing systematic 
bud selection in the commercial propagation of those plants in which it 
has been proved to be an effective method of improvement. The efficacy 
of bud selection depends upon the nature of bud variation. 

Bud Variation in Plants. — There are two kinds of bud variations, 
viz., modifications and mutations. Modifications are common to all 
plants and are easily detected even in dormant buds. On deciduous trees, 
for example, the buds formed during one season's growth usually show 
considerable variation in size. Such variations do not necessarily repre- 
sent inherent differences between the buds. They are usually due to 
differences in the particular combinations of conditions which exist during 
development of the buds. Phytomers exhibit fluctuating modifications 
in all other characters as well as size in response to the varying con- 
ditions of nourishment, light, temperature and other elements of the 
environment. These modifications are not transmissible and selection 
of such bud variations alone could never change the average output of 
an orchard or establish an improved strain. 

Bud mutations, on the other hand, although comparatively rare, are 
of general occurrence and the new characters induced by them are trans- 
missible. Hence in considering the efficacy and practicability of bud 
selection in a horticultural variety the first thing to be determined is the 
nature and frequency of somatic mutations in that variety. There is 
only one way in which this question can be answered completely and 
definitely and that is by extensive tests of vegetatively propagated off- 
spring. Such tests must be made under controlled conditions especially 
as regards the nature of the rootstock on which the tested scions are pro- 
25 385 



386 GENETICS IN RELATION TO AGRICULTURE 

pagated. Careful inspection may reveal a certain number of chimeras 
and bud sports. Both, in fact, are comparatively common in some 
varieties, such as the Boston Fern and the Washington Navel Orange, 
and they sometimes give rise to superior new biotypes. Yet inspection 
alone is not sufficient. A new fern sport must be propagated in order to 
test its constancy when multiplied vegetatively. Similarly a supposed 
orange sport must be propagated and the progeny must be tested in 
order to ascertain whether the selected phytomer is really a mutation 
and a desirable one at that. 

Having discovered a new type which originated by bud mutation, 
the question arises : Will there be any practical difficulty in maintaining 
this new form by means of vegetative propagation? Are additional 
somatic mutations likely to occur in sufficient number to endanger the 
preservation of the selected form? It will be remembered that sometimes 
bud mutations in the Boston Fern produce ever-sporting varieties that 
have little or no commercial value. Similar inconstant forms have 
arisen in other ornamentals. Nevertheless it has been practicable 
to propagate vegetatively many valuable bud sports and hybrids, in- 
cluding some that are highly variable. One such product of composite 
hybridization is the cultivated Coleus. This ornamental foliage plant 
is commonly used for beds and borders in summer and as a conserva- 
tory plant in the colder months. The foundation stock was produced 
in England about 1867 by hybridizing four different exotic species. 
Hundreds of named varieties have been produced, some having appeared 
as bud mutations, but the majority being seedlings. Some of the varie- 
ties now in cultivation are characteristically variable. In one such 
variety Stout has investigated the variations in leaf color pattern and 
leaf shape in a series of 833 plants, all descended by vegetative 
propagation from two similar plants. 

Bud Selection in Coleus. — The two plants with which Stout began 
had a definite pattern of leaf coloration consisting of a green mid-region 
and yellow border with blotches of red in the epidermis (Fig. 159). The 
green and yellow pigments exist in the sub-epidermal layers. The 
vegetative offspring from the two original plants were kept separate and 
the simple habit of branching in this plant made it possible to indicate 
the particular branch as well as the individual plant from which a cutting 
was taken. In this way Stout was able to trace the pedigree of any plant 
to its original source. During the course of the investigation 16 new 
color patterns were obtained. There also appeared the laciniate form 
of leaf which is seen in the younger leaves of the plant on the left in Fig. 
159. Of the 16 new color patterns 15 arose by somatic mutations which 
produced bud sports either directly or, in some cases, indirectly from 
chimeras. The other new pattern arose solely as a fluctuating variation. 



BUD SELECTION 



387 




Fig. 159.— ^Two Colcu.s plants which descended from the same branch, which at the 
time it was propagated was uniform as to leaf shape and bore leaves having the same 
general type of color pattern, viz., green mid-region and yellow border with red blotches on 
the epidermis. The right hand plant resembles the original plant although it represents 
the fourth vegetative "generation." The left-hand plant shows the laciniate type of leaf 
which appeared several times as a bud mutation. (From Stout.) 

Yellow- Red 
/Blotched 



Green- Yellow 
Ked Blotched 





Spontaneous Yellow 

—5 7 --aor4auciE 5 and 

'8a Green-Yellow Spotted-Solid .Eed 
13a Green-Yellow Spotted 

Spontaneous Yellow 



id E 



■9 Greeu-Solid Red 13n 

■13 Green 13a 

1 



14 Yellow-Green 

9 Green-Solid Ked ^^ ""^^ ^ 

10 Green- Yellow-Solid Eed Upper Center 
ll-Yellow- Green-Solid Red 



ICYellow. Solid Red 
'l2 Green-Yellow 

Fig. 160. — Diagram showing derivations of color patterns in Coleus. The descriptive 
name of a color pattern is given only whore it first appears in a line of descent. A contin- 
uous line indicates origin by bud mutation and a dotted line indicates fluctuating variation. 
(No. 2 = original form. No. 7 = laciniate-leaved form. E = entire leaves.) (From 
Stout.) 



388 



GENETICS IN RELATION TO AGRICULTURE 



It consisted of absence of yellow and decrease of red in the younger 
leaves of a few plants. Six of the 15 patterns that arose as bud mutations 
also appeared more or less frequently as fluctuating variations on certain 
plants. The derivations of the various color patterns are shown by the 
diagram in Fig. 160. It will be noted that the original pattern, No. 2, 
is described as green-yellow-red blotched which means green center, 
yellow marginal border and red blotches on the epidermis. In the same 
way the description of pattern No. 4 is interpreted as green center, yellow- 
spotted marginal border and red blotches on epidermis. 

The frequency with which these various bud mutations occurred is 
shown in Table LIV. Here are indicated under ''Plants" the number 
of plants in which each type of change might have occurred, under 
"Frequency," the actual number of bud mutations that did appear, and, 
finally, the ratio of bud mutations to constant buds. This ratio is 
obtained by assuming that on the average each plant produced 200 buds. 

Table LIV. — Frequency op Bud Mutations Producing New Color Patterns 
AND Leaf-shapes in Coleus. {After Stout.) 



Type of change 



Plants 



Frequency 



Ratio 



Increase of yellow and decrease of green . 
Decrease of yellow and increase of green . 
Reversal of positions of green and yellow 
Increase of epidermal red to solid red . . . 
Decrease of epidermal red, complete loss 

Decrease of epidermal red, all cases 

Appearance of the laciniate character . . . 
Entire leaf from laciniate leaf 



827 


27 




740 


50 




450 


8 




770 


8 




815 


19 




815 


21 




765 


13 




68 


1 





6,130 

2,960 

11,250 

19,250 

8,580 

7,760 

11,770 

13,600 



Stout remarks that these data indicate the tendencies of the bud 
variations and give a clew to the behavior of the characters in question. 
Thus, decrease of yellow occurred twice as often as increase of yellow, 
and loss of red 2.2 times as often as increase of red. Although these 
data indicate a tendency toward loss rather than gain of the two colors, 
the fact that the number of mutations involving gain is about half as 
large as the number involving loss has considerable interest. It has been 
generally considered that mutations involving addition of a character are 
exceedingly rare. While this may be the case in many pure species, it 
would appear from the above evidence that among the progeny of species 
hybrids such mutations may be relatively frequent. 

The manner of appearance of these bud mutations was typical of 
somatic factor mutations. Stout says, for example, "the loss of yellow, 
loss of green, and gain and loss of red all occurred in single branches and 
in sections of branches. Frequently two quite different changes occurred 



BUD SELECTION 389 

on the same plant." After citing a case of modification in degree of 
pigmentation in Coleus by the use of artificial light, Stout declares: ''In 
marked contrast to these results it may be noted that the bud variations 
that I have reported give more marked changes than those induced by 
Flammarion and that these appear suddenly and in a sector of a bud in a 
manner that suggests internal readjustments rather than external 
environmental influence." 

Stout tested the seed progeny of two of his plants, obtaining in all 45 
plants from selfed seed. As would be expected in such a case of composite 
ancestry, there was a wide range of variation in size as well as shape of 
leaves and in leaf coloration every gradation between pure yellow plants 
that died within a few weeks and pure green. No better evidence could 
be offered that these highly variable characters are actually conditioned 
by specific factors which segregate and recombine in sexual reproduction. 

The history of Coleus as reviewed by Stout also contains some interest- 
ing facts about the origin of the characters under discussion. The four 
original species that furnished the foundation stock, from which the 
modern Coleus has been developed, contained no yellow coloration 
whatsoever. They were characterized by green leaves overlaid with 
different shades of red, purple or chocolate. The first appearance of 
yellow occurred in a leaf sport, i.e., in one-half of a single leaf the green 
was exchanged for a decided yellow tint. "The bud at its base was 
propagated and gave the new variety." However, it does not appear 
that this sport was used in hybridization work. "Yellow coloration also 
appeared as a new or spontaneous development" among the second lot 
of hybrids raised at the gardens of the Royal Horticultural Society about 
1869. Again in regard to the laciniate-leaf character, as early as 1856 a 
variety of Coleus hlumei (which was the first species introduced into 
Europe and was originally described in 1826) was described as being 
''somewhat more richly colored but differing chiefly in having the leaves 
deeply and doubly lobed." While there is no record of the origin of this 
variety, it is certain that it appeared before C. hlumei had been hybridized 
with any other species. It is very probable, therefore, that it arose as a 
bud mutation. Thus it appears that two of the three characters whose 
presence, absence or partial development figure conspicuously in Stout's 
"bud variations" actually originated by factor mutations during the 
early horticultural history of this plant. 

Regarding the efficacy of selection in maintaining the new forms that 
arose by bud mutation, Stout's results show clearly that even in such a 
highly variable plant as a horticultural variety of Coleus, bud selection 
is very effective. Stout describes his methods as follows: 

"The series of plants considered under any type jiattern are in large measure 
a selected stock. When cuttings were made for the perpetuation of the pattern 



390 



GENETICS IN RELATION TO AGRICULTURE 



in a new generation, they were made from the plants most typical and constant 
for the pattern concerned. When a bud variation appeared, if the conditions 
were favorable, the parts possessing it were allowed to develop until there were 
several branches from which cuttings could be taken simultaneously. In such 
cases the selection of branches for the new type was a simple matter, as it 
depended on the talcing of branches sharply distinct from the main part of 
the plant. . . . When further cuttings were made for a new generation to 
perpetuate the type they were made from plants most uniform and constant 
(determined from the records) for the pattern in question. Usually but three 
cuttings were taken from a plant and these were taken from branches most uni- 
form and clearly conforming to the type." 

The relative numbers of "constant" plants and of plants showing 
either clear-cut bud mutations or ''fluctuations" are of considerable 
interest in connection with this matter of maintaining strains. These 
proportions are given in t\vQ tabulated summary of the main clones that 
were derived from one of the two original plants Table LV. Clones 11 

Table LV. — General Summaky of Clones Derived prom Plant No. 1 



Clone 


Total 

number of 

plants 


Plants 
constant 


Per cent, 
of plants 
constant 


Number of 
bud variations 


Ratio of 
frequency 


11 

12 

13 

14 

117 

111 


211 
192 
138 
155 
91 
34 


132 

87 
75 
80 
54 
29 


62 
45 
54 
51 
59 
85 


49 
21 

4 
18 
31 

4 




860 
1,830 
6,900 
1,720 

590 
1,700 



and 12 were derived from two branches that had the same color pattern, 
yet it seems that they possessed "quite different potentialities for con- 
stancy and for bud variations. Even more marked differences than these 
developed among the various subclones. A study of pedigrees shows that 
in all patterns and in all main clones there were certain lines of progeny 
much more constant than many others. These could not be detected by 
any other than a pedigree method." These facts have a definite bearing 
on the maintenance of yegetatively propagated varieties or strains. If 
similar diversity as regards degree of variability exists in other cultivated 
plants, as it undoubtedly does, careful bud selection must be of prime 
importance in keeping varieties true to type or at least in preventing 
deterioration through the accumulation of undesirable bud mutations. 
In Coleus at least it is certain that bud selection is effective and necessary 
in maintaining strains true to type. If it is equally potent in its effect 
on other vegetatively propagated plants, bud selection should be given 
far more attention by nurserymen than it has generally received. 



BUD SELECTION 391 

Bud Selection in Horticultural Practice. — The probable importance 
of bud selection to American pomology was recognized by Munson in 
1906. He advocated its practice in the following words: "Select through 
successive generations buds, that is cuttings or scions, from branches 
which bear fruit most nearly approaching the ideal." Two years later 
Webber presented the subject of ''clonal or bud variation" to the 
American Breeder's Association and also recommended bud selection. 
At this time, however, there was but little evidence that could be cited 
as proof of the practical value of bud selection. Working with violets 
Galloway and Dorsett were able to produce tlisease-resistant and florif- 
erous strains. The Boston Fern sports were beginning to attract at- 
tention. But it was still generally supposed that bud sports were of 
comparatively slight importance as far as pomological practice was 
concerned. About this time two of the pioneer citrus growers of Cali- 
fornia, E. A. Chase and J. P. Englehart, became interested in the numerous 
variations among their orange and lemon trees and the latter began to 
experiment with bud selection. He first recognized and propagated the 
Golden Nugget Navel Orange, a sport from the Washington Navel. 
He soon convinced himself that many of the variations in fruit characters 
which he observed could be perpetuated by budding. In 1910 Coit 
emphasized the fact that through the unintentional propagation of 
undesirable sports a gradually increasing proportion of the trees in the 
citrous orchards of California were developing into drones or worthless 
types and that the only way to prevent this was greater care in choosing 
the buds used in propagating nursery stock. Meanwhile, Shamel had 
begun an extensive series of experiments on the improvement of citrous 
fruits through bud selection. During his preliminary investigation in 
1909 Shamel found what he took to be distinct types of Washington 
Navel Oranges and the observations and experiences of certain growers 
seemed to indicate that trees producing small annual yields and poor 
quality of fruit could be top-worked with scions from trees known to 
be annual high producers of superior fruit to the very great advantage 
of the citrous fruit growers. Accordingly an elaborate system was de- 
vised for recording the performance of individual orange, lemon and 
pomelo trees. 

Performance Records as a Basis for Bud Selection. — This study of 
the performance of individual citrous trees has emphasized the fact that 
there may be inherent differences with respect to quality and yield be- 
tween different trees of the same commercial variety. Already these 
tests have been supplemented by demonstrations in top-working low- 
producing trees with scions from fruitful individuals. As a result of the 
systematic campaign which the government representatives have con- 
ducted throughout the citrous fruit districts of California and Florida, 



392 



GENETICS IN RELATION TO AGRICULTURE 



many of the growers are keeping records of the annual yields of part 
or all of the trees in their orchards. 

Bud Mutations in Citrus. — What has been accomplished through bud 
selection in citrous fruits has been made possible by the relatively high 
frequency with which bud mutations occur. A dozen distinct types of 
Washington Navel orange are now known to occur more or less fre- 
quently in California orchards (Fig. 161): This fact is of especial sig- 
nificance in the light of the history of the Washington Navel orange which, 
as it originally existed in Southern California, consisted of only a few trees 




Fig. 161. — Fruits of the Washington Navel orange (1), and four forms that have 
originated from it by bud mutation; (2), Thomson Navel; (3), Yellow Navel; (4), Corru- 
gated; (5), Ribbed. {Courtesy U.S. Department of Agriculture.) 

(possibly only two) which were propagated from navel orange trees 
that were introduced from Brazil by the U. S. Department of Agriculture. 
According to Coit the evidence from early California horticultural 
literature indicates that the Washington Navel variety was recognized 
as a distinct and at least fairly uniform type of orange. That a strong 
tendency to mutation characterizes this variety is evidenced by the 
frequent origin of new forms or reappearance of old ones as bud sports. 
In some cases the aberrant type differs not alone in fruit characters but 
also in habit of growth or leaf-shape and frequently in yield. In fact high 
yield is said to be correlated with superior fruit at least in some types. 
Similarly in the Eureka lemon the so-called "shade-tree type" makes 



BUD SELECTION 393 

rank growth and low yields, and such trees have been successfully top- 
worked with scions from fruitful types. 

Deciduous Tree Fruits. — The efficacy and practicability of bud se- 
lection in other species than the citrous fruits is not yet determined. But 
there is considerable evidence that in certain varieties at least it is 
possible to find distinct types which remain constant when propagated 
vegetatively. In the apple and peach bud sports are known and they 
may be more frequent than has been supposed. Dorsey mentions four 
varieties of apple which originated in this manner and reports the dis- 
covery of another sport. It is possible that certain varieties have a 
greater tendency to sport than others. The Baldwin apple may be such 
a variety. It is claimed by some authors that the many variations occur- 
ring in this variety are purely environmental, while others assert that they 
have propagated such variations successfully. But in the apple and most 
other deciduous fruits there are plenty of good varieties which are adapted 
to conditions in the regions at present devoted to fruit growing. Here, as 
with citrous fruits, new varieties are not needed so much as profitable 
orchards. Will it pay to keep performance records as a basis for bud 
selection in deciduous fruits? That is the critical point, and it is not yet 
settled. Both favorable and adverse evidence has been presented. The 
Virginia Station kept a record for four years of the yields of 1245 trees in 
the same apple orchard (variety or varieties and age not known). Of 
these 375 yielded an average of four barrels to the tree and produced 60 
per cent, of the crop, while 215 trees averaged less than one barrel per tree 
and were kept at a loss. The Dominion Experiment Station, Ottawa, 
Canada, has kept records of yields of different trees in the same orchard 
covering a period of 18 years. The most productive tree of McMahon- 
White yielded 1250 gal., and the least productive, 882 gal. Of Patten 
Greening the most productive tree yielded 974 gal., and the least produc- 
tive 586 gal., while in the case of Mcintosh Red one tree yielded 1219 gal., 
and another 670 gal. Clearly there are wide differences in the produc- 
tivity of individual apple trees. Much of this variability in production 
is probably due to soil differences. It is claimed by Powell, however, that 
one cause of the marked decrease in the number of apple trees in New 
York State is the absence of profits due to low-producing trees. The true 
condition can be determined only by keeping performance records on an 
extensive scale. Even though many healthy but low-producing trees 
may be found, there still remains the question whether or not it will pay to 
top-work these low-producers with scions from high-producers. It has 
not yet been determined whether any of the wide differences in the 
productivity in individual apple trees are due to bud mutations. 

Next to yield, uniformity of fruit is perhaps the most important 
commercial consideration. But there are marked differences in varieties 



394 GENETICS IN RELATION TO AGRICULTURE 

in respect to uniformity and in the extent to which this feature is trans- 
mitted to vegetative offspring. The experience of Tyson Brothers in 
Adams County, Pa., illustrates this point. They propagated 8000 trees 
with scions from two old trees of York Imperial apples which had been 
noticed because of their productivity and the uniformity in shape of 
their fruit. Unfortunately no scions were taken from average or poor 
trees, and hence there is no basis for comparing productiveness in the young 
orchard. Furthermore it is possible that these trees will exhibit less 
variation in form of fruits with increased age. But, as yet, so far as 
uniformity of fruit is concerned, the experiment seems to have been 
decidedly inconclusive. Progress with a similar experiment on the Ben 
Davis variety has been reported by Whitten. Scions were taken from an 
exceptionally poor tree and from another tree which produced the largest 
and best apples of its kind on the station grounds. Examination of the 
third year's crop showed no perceptible difference in size, color, grade or 
quality of the fruit from the two lots of trees. In fact the average yield 
per tree was somewhat higher in the lot propagated from the poor tree 
than in the lot propagated from the superior individual. There appeared 
to be as much variation between individual trees in each plot as between 
the two plots. 

"Pedigreed" Nursery Stock. — In response to the growing interest in 
bud selection many nurserymen have taken advantage of the idea of value 
which is commonly associated with pedigree. The more conscientious 
ones have selected their scions from trees which they believe to be supe- 
rior, but a certificate of source is not a pedigree. This term, it must be 
admitted, has been used in scientific investigations of vegetatively 
propagated plants, where careful records were kept for a relatively 
large number of asexual "generations," as in Stout's work on Coleus. 
But no nursery stock now on the market is entitled to be known as pedi- 
greed and even though such stock may be produced in future years, 
the danger from misrepresentation, either intentional or unintentional, 
will be as great as ever. Coit has suggested that stock propagated from 
tested trees be known as recorded stock and recommends a simple plan 
by means of which Deputy County Horticultural Commissioners may 
officially seal and record each tree when it is budded. 

Bud Selection in the Potato. — No one doubts the occurrence of bud 
mutations in the potato. Numerous instances of the origin of new 
varieties as bud sports are on record. Yet there is considerable difference 
of opinion regarding the relative frequency of bud mutations in this 
species. Numerous investigations have been made on the improvement 
of the potato by means of tuber and hill selections. The most important 
papers have been reviewed by East and, more recently, by Stuart. East 
observed over 700 named commercial varieties during a period of 3 



BUD SELECTION 



395 



or 4 years and found 12 bud mutations. Changes were noted in the 
color, shape and habit of growth of the tubers and in the depth of the eyes. 
But as for the bearing of bud mutations on origin of new varieties East 
reached the conckision that, while isolated cases of improvement might 
be due to selection of bud mutations, yet comparatively few (probably less 
than 0.5 per cent.) of our present varieties arose in this manner. This 
evidence on the origin of varieties has led East to adopt the view that 
probably all bud mutations are so exceedingly rare in the potato that few, 
if any, cases of "running-out" or "degeneration" in varieties are to be 
explained on this basis. He beheves the principal factor in such dete- 
rioration is disease, and that in numerous experiments on potatoes, in 




Fig. 162. — Variation in yield between tuber-units from the same hill. Above, the 
progeny of two tubers from hill selection No. 35; below, that from hill selection No. 4. 
(After Stuart.) 

which it is shown that successive selections have raised the average 
yield over that of the unselected tubers, the results are entirely due to the 
elimination of diseased tubers. 

While the ehmination of diseased tubers or of tubers that were weak- 
ened by disease in the leaves or stem does undoubtedly explain the success 
of many selection experiments it may not account for all of them. Tests 
of individual tubers of almost any commercial variety apparently reveal 
inherent differences in the tubers. Although the plant is very susceptible 
to environmental conditions and some tuber characters such as shape and 
size are very unstable, yet sometimes the product of two closely similar 
tubers which came from the same hill when grown under closely similar 
conditions will differ widely (see Fig. 162) . The most satisfactory method 
of testing individual tubers is the tuber-unit method which was introduced 
by Webber. Each tuber which is to be tested is cut lengthwise into 
four equal pieces which arc planted at equal distances from each other. 
The four hills thus comprise a tuber-unit. 



396 



GENETICS IN RELATION TO AGRICULTURE 



Stuart in 1911 conducted a tuber-unit experiment with some 150 
standard commercial varieties of potatoes. 

"The seed used was grown in Burhngton, Vt., in 1910, on land which had not 
grown a cultivated crop of any kind for at least 35 years. In addition to 
this the seed was selected from the most promising hills at the time the 
crop was harvested. The tubers as a whole were remarkably uniform in 
size and there could, therefore, have been little difference in the size of 
the seed pieces used. Any variation, therefore, which occurred between the 




flBHy^Ml 4li Mill 



Fig. 163.^Strong and weak tuber-units of the Gold Coin variety of potatoes. Nos. 1 
and 2 represent strong and weak tuber-units in 1911; Nos. 3 and 4 represent yields from 
tuber-units 1 and 2; Nos. 5 and 6 represent yields in 1912 from 5 tuber-units of Nos. 3 and 
4. {After Stuart.) 

plants of the various tubers which were planted would seem to be due to 
some inherent tendency in the tuber itself. The remarkable dissimilarity 
between the growing plants of the individual units of a variety planted 
contiguously in the row was so surprising that some three dozen units were 
photographed and when these were harvested the tubers were also photographed 
(see Fig. 163). It was found that the divergency in yield was just as great as 
in the size and vigor of the plants. In 1912 five units were planted from both 
strong and weak plants, and it was found in practically every instance that the 
low-yielding 1911 plants gave poor germination, a feeble vine growth and a still 
lower yield than in 1911." 

There have been many experiments similar to the one above de- 
scribed and they certainly indicate that a certain proportion (from 5 to 



BUD SELECTION 397 

10 per cent, according to Stuart) of weak, diseased or unproductive 
plants are to be found in all unselected varieties. It is equally certain 
that in the vast majority of cases in ordinary field practice these are 
unrecognized and the resultant effect upon yield is unnoted. Even 
though lack of vigor and low yield is entirely due to disease and hill or 
tuber selection does nothing but eliminate these undesirables, it will be 
well worth doing. But other characters, as well as yield and vigor, 
should be kept in mind in attempting to produce an improved strain of 
potatoes. The ideal market tuber is of medium size, round or oblong in 
outline and somewhat flattened. The eye should be shallow. The 
eating quality should not be overlooked, but varieties and tastes differ 
greatly in this respect. In addition to these, adaptability to local 
conditions and disease resistance should receive attention. 

"The selection of a large number of high-yielding hills which are then thrown 
together for mass planting the ensuing year is not likely to result in any marked 
improvement except by the elimination of the diseased or unproductive plants. 
The only certain method of securing a superior strain is to plant each selection 
separately. . . . Every progressive farmer should have his selection plot, 
in which to grow his yearly selections; and, in addition, he should have his in- 
crease plot, where the promising selections may be increased for the field-crop 
planting" (Stuart). 

Certified Seed Potatoes. — ^The certification of seed potatoes based on 
official inspections during the growing season and after harvesting has 
been adopted in some states. According to Milward the summer in- 
spection considers stand, vigor of vine, specific and non-specific diseases 
and varietal purity; and bin inspection looks after conformity to type, 
diseases, market condition, quality and yield. In view of the increasing 
importance attached to disease in the degeneration of potato varieties 
some such system of inspection and certification should be adopted in 
every state where potatoes are extensively grown. But it must be borne 
in mind that complete protection against failure or loss is by no means 
assured even under a system of seed certification. Stewart has recently 
reported several instances of sudden degeneration of prohfic strains, 
at the Cornell University Experiment Station, through the invasion of 
some obscure disease of which there are a number that infest the potato. 
In some cases only the larger tubers in a hill are alTected while the smaller 
tubers are apparently healthy. Stewart's conclusions follow: 

"(1) Neither normal foliage nor high yield is a guaranty of productivity in 
the progeny of the following season. Degeneration may occur quite suddenly. 
(2) It is unsafe to select seed potatoes from fields containing many degenerate 
plants. Even the normal plants from such fields are liable to produce worthless 
progeny. (3) Mosaic threatens to become an important factor in the production 



398 GENETICS IN RELATION TO AGRICULTURE 

of seed potatoes. It is transmitted through the seed. (4) It is doubtful if any 
method of seed selection will prevent the "running out" of seed potatoes under 
certain conditions." 

Oth€r Crops in Which Bud Selection May Apply, — It is claimed that 
many desirable varieties of roses, carnations, chrysanthemums, violets 
and other plants which are cultivated for their flowers originated as 
bud sports. The best florists are very critical regarding the character- 
istics of their stock and sports are soon discovered. The importance of 
propagating from typical, healthy plants is generally appreciated. 
Many of the roses used for forcing winter blooms produce two types of 
shoots which are known to the horticulturist as blind and flowering 
wood. For some years the Bureau of Plant Industry conducted ex- 
periments on the selection of buds from the two types of shoots, but it 
became apparent that the diversity among individual plants in regard 
to their flowering habits, whether propagated from blind or from flowering 
wood, was greater than the diversity between the progeny of flowering 
wood plants as compared with the progeny of blind wood plants. As a 
result of fertilizer experiments with the variety, My Maryland, Blake 
inferred that there was a real basis for production of improved strains 
by bud selection. But he points out that it would require time and much 
care in selection and that the average florist can hardly attempt to do 
more than to note the relative vigor of his plants at various stages and 
propagate from the best producers that are not especially favored by 
particular environmental conditions. 

The strawberry is so important commercially and comes into bearing 
so soon when propagated from offsets that if bud selection were effective 
in producing improved strains it would be of tremendous practical value. 
But the results of experiments indicate that the individual differences so 
frequently observed in strawberry plants are merely modifications. Whit- 
ten reports that bud selection of strawberry plants during a period of 
15 years has given no gain in the total productiveness of the plots 
which originated from high-productive plants over the plots which 
originated from low-productive plants of the same variety. The experi- 
ment began by selecting six plants that yielded four times the amount of 
fruit of six low-producing plants all of the Aroma variety. Each 
succeeding year selections in the high-yielding plot were made from the 
highest plants and in the low-yielding plot from lowest-yielding plants. 
It is possible that in some varieties of strawberries bud mutations occur 
more frequently than in others. But in order to find a high-yielding 
plant whose high-producing character would be maintained among its 
vegetative offspring it would probably be necessary to test hundreds of 
individual high-producing plants. Hybridization offers much greater 
promise in the production of high-yielding strains of strawberries. 



liUl) SELECTION 399 

Limitations of Bud Selection. — The efficacy of bud selection as a 
means of improving the type is dependent upon the occurrence of bud 
mutations; its practicability, upon their frequency. As a method of 
plant improvement bud selection will always be handicapped because 
recombinations of factors are possible only in sexually reproduced 
individuals. Moreover, it appears that in some vegetatively propagated 
crop plants desirable bud nuitations, which can be detected without 
resorting to statistical methods, are so rare that bud selection can never 
become a generally used method of producing new varieties, even though 
it may occasionally be used effectively for that purpose. On the other 
hand, it is highly practical to give careful attention to the selection of 
scions from such plants as are known to be healthy and typical of the 
variety. Such bud selection is a means of preventing the propagation 
of worthless or undesirable mutations and it should be practised by 
every nurseryman as a matter of course. 



CHAPTER XXIV 
BREEDING DISEASE-RESISTANT PLANTS 

The term, plant disease, has been restricted by some authors to those 
disorders and abnormaHties caused by fungous parasites only. Other 
authors have employed the term in a more general sense, including there- 
under all abnormal conditions of structure and function which are caused 
by the different elements of the environment. We shall use the term in 
this more general sense and for the purpose of this discussion it may be 
defined as follows. Plant diseases include all the ailments and injuries 
which can be traced to specific causes or agencies as well as certain func- 
tional disorders the causes of which are obscure or difficult of analysis. 
In order to discuss profitably the breeding of disease-resistant plants it 
is necessary to consider more fully the various categories of causes. 

The Causes of Plant Diseases. — In general the diseases of plants are 
caused either by unfavorable conditions among the inanimate elements 
of the environment or by the invasions of other organisms. While every 
case of disease must be considered as the result of interrelated causal 
agencies, yet it is usually possible to discover specific agents that are 
primarily responsible for the pathological condition. It is then possible 
to determine the nature of disease resistance in particular instances with 
more or less definiteness according to the nature of the specific causes. 

The most important non-living elements of the environment affecting 
the health and vigor of cultivated plants are the soil, the water supply and 
the temperature and humidity of the atmosphere. These environmental 
factors influence plant development in so many ways that the oppor- 
tunities for maladjustment between plants and their environment are 
practically endless. Such conditions as excess of alkali or lack of suffi- 
cient moisture in the soil or the combination of excessively high temper- 
ature and low relative humidity are typical and important illustrations 
of specific environmental conditions which induce disease in plants. 

The living organisms of chief importance in causing plant diseases are 
insects, fungi and bacteria. Injurious insects may be roughly classified 
according to their ways of feeding under two heads, viz., sucking and 
biting insects. The first class includes the plant lice, phylloxerans and 
scale insects which obtain their nourishment by sucking it from the living 
plant. The second class includes all moths and butterflies whose larvae 
devour living plants as well as beetles and other insects that obtain their 

400 



BREEDING DISEASE-RESISTANT PLANTS 401 

food in similar fashion. Pathogenic fungi and bacteria are wholly or 
partially parasitic. Bacteria which cause plant diseases are those capa- 
ble of establishing themselves and multiplying in number within the 
living tissue of the host, A few of the important plant diseases caused 
by bacteria are "fire-blight" of pears and apples, crown gall of many 
fruit trees, grapes and other plants, and the black rot of the cabbage. 
Some fungi, such as rusts and smuts, are strictly internal obligate 
parasites (as distinguished from those obligate parasites which are wholly 
or partially epiphytic), i. e., they cannot exist outside the body of a 
particular host plant or plants except in the spore stage. In such cases 
the relation between parasite and host is symbiotic. The specific re- 
lations between parasites and their hosts vary from a condition of tol- 
erance of the parasite without serious injury to the host to one in which 
the destruction of the host finally ensues. Many fungi, such as the 
powdery mildews, are epiphytic although they derive their nourishment 
from the living plant tissue by means of haustoria. Between the epi- 
phytes on the one hand and the internal parasites on the other are many 
types of endophytic fungi in which various proportions of the parasite's 
life cycle are spent within the host plant, 

Thiis there are many agencies, some non-living as well as many living 
things, which threaten the normal development of cultivated plants. 
Even among the parasitic fungi themselves there are many devices for 
invading the host plant and many instances of specific physiological 
relationship between parasite and host. 

The Nature of Disease Resistance in Plants. — Disease resistance in a 
plant may be defined as the ability to develop and function normally 
under conditions such that other plants of the same species fail to develop 
or are destroyed. Resistance is always either partial or complete. The 
avoidance of disease by such means as precocious or delayed maturity is 
hardly to be considered as true disease resistance. Since there are so 
many agencies which may cause disease in plants it is evident that the 
ability to resist disease may depend on any one of many characters or it 
may involve every function of the plant. In either case this ability is a 
manifestation of the physiological individuality of the plant and hence it 
may be inherited. Nowhere is this more strikingly shown than in the 
disease resistance of certain natural species. 

Disease Resistance in Natural Species. — The nature of disease resist- 
ance in a particular instance is indicated by the nature of the cause of the 
disease. In the case of non-living causes resistance on the part of certain 
plants can be explained only as a manifestation of the inherent properties 
of the protoplasm. Thus the alkali resistance of salt grass, the Austrahan 
salt bushes, the common beet and asparagus is a heritable character. If 
it were not so these species could not perpetuate themselves on soils which 

26 



402 GENETICS IN RELATION TO AGRICULTURE 

are too strong in alkali content for most plants. Similarly with many 
plant troubles that are referred to adverse soil conditions, such as chlorosis 
and die back, it has been found that some species are much better able to 
resist such conditions than other species and within a particular species 
certain varieties may be more resistant than other varieties. This holds 
true in the case of other non-living agencies such as excess and deficiency 
of moisture and heat. For every plant there is a set of optimum condi- 
tions and these conditions are very different in different species and among 
varieties of the same species. For example, rice flourishes in standing 
water while maize requires well aerated soil. But there are thousands of 
varieties of rice, each one adapted to the conditions peculiar to a certain 
locality and there are many varieties of maize which make possible the 
culture of this species under conditions varying from the humid corn belt 
to the arid regions of northern Mexico, Bolivia and central China. 
Similarly in other field crops and in fruits, in various parts of the world 
there exist species and varieties which are adapted to certain local condi- 
tions that would be inimical to normal development of related species and 
varieties. Agricultural exploration cooperating with systematic seed and 
plant introduction has already made available for the plant breeder a 
large number of distinct forms of economic plants which in course of time 
may revolutionize many productive and manufacturing industries. 

Turning now to the phenomena of resistance to the attacks of animal 
or plant parasites, we find that natural species are characterized by as 
great diversity in this respect as was observed in the case of resistance to 
alkali, drouth and other physical elements of the environment. A few 
specific examples will serve to illustrate this general principle. The 
relation of different species of the grape to the phylloxera, Peritymhia 
vitifolice Fitch {Phylloxera vastatrix Planchon), is representative of a great 
number of reported instances of insect parasitism on vegetation. Also 
in their general aspects the phenomena of variation in phylloxera resist- 
ance among species of the vine are representative of the facts of disease 
resistance in general. Moreover, on account of the great economic 
importance which this particular vine disease assumed in Europe some 
forty years ago, and later in Calif o-rnia, there has been a large amount of 
investigation on the culture of grapes in phylloxera infested regions. The 
life cycle of this insect includes both leaf-feeding and root-feeding forms. 
The extent of the injury caused by the warty galls on the leaves is com- 
paratively insignificant. It is the root-feeding form which inflicts serious 
damage to susceptible vines. On the roots of such vines the character- 
istic symptoms are of two distinct kinds, viz., small galls or "nodosities" 
near the tips of young rootlets, and larger swellings or "tuberosities" 
occurring upon the older rootlets and roots (Fig. 164) . The root-tip galls 
or nodosities are commonly found even on resistant species if phylloxera are 



BREEDING DISEASE-RESISTANT PLANTS 



403 



present. The principal diiTerence between resistant and susceptible vines 
as regards reaction to phylloxera attacks is found in the number, size and 
penetration of the lesions on the larger roots. This phylloxeran is a 
native of eastern North America and the species of Vitis which are native 
to this region all exhibit some resistance to its attacks. This resistance 
of species native to the habitat of a disease-causing parasite is a general 
fact of great significance to agriculture on account of its potential value 
in both plant and animal breeding. 




Fig. 164. — Effects of phylloxera on vine roots. On left affected root tips or nodosities; 
in same figure incipient tuberosities are shown at a. Center, non-penetrating tuberosities 
on an American vine. Right, penetrating and confluent tuberosities on V. vinifera, the 
most serious form of the disease. {After Viala and Ravaz.) 

• 

The phylloxera was introduced into France through the importation 
of American vines and it soon became a most serious obstacle to the 
culture of the choice wine, table and raisin grapes of the Mediterranean 
region, all of which varieties belong to a single species, Vitis vinifera. 
In fact, every member of this large and valuable plant group has been 
found to be susceptible to phylloxera thus making impossible its culture 
as a direct producer, i.e., on its own roots, in a phylloxera infested region. 
After striving in vain to exterminate the insect in all infested areas, 



404 GENETICS IN RELATION TO AGRICULTURE 

European vineyardists gradually adopted the only other practicable 
method of grape growing, viz., the grafting of vinifera varieties upon 
resistant roots. The problem of determining which species of Vitis were 
both highly resistant to phylloxera and well adapted to the soil and 
climatic conditions of various European localities required extensive 
investigations. Eighteen native American grapes have been tested as 
well as several Asiatic species, but the latter were all less resistant than 
the most susceptible American species. The American vines which 
have come into most prominence on account of their proven value in 
the reconstitution of phylloxera devastated vineyards may be listed 
according to relative resistance about as follows, if the maximum or absolute 
inimunity be taken as 20. 

18-19. V. rupestris. 

18. V. riparia and cordijolia. 

17. V. herlandieri. 

16. V. cinerea. 

14-15. V. cestivalis, linsecomii and candicans. 
All of the above species belong to the sub-genus or section, Euvitis. 
Two of these, rupestris and riparia, together with certain hybrids 
between these and between these and vinifera, are now considered the 
most valuable resistant stocks. Another American species belonging to 
the section Muscadinia, viz., rotundifolia, has been found to have a 
resistance of 19 or higher inasmuch as the insect has never been observed 
on its roots. It is also free from the common fungous diseases of the 
vine, but the difficulty of propagating it from cuttings and its slight 
affinity for grafts of other species make it a valueless species for the 
reconstitution of vineyards. On the other hand, the American species, 
labrusca, has become of great economic importance since it is the parent 
of the Concord, Isabella, Niagara and many other cultivated varieties. 
Yet its resistance to phylloxera is ranked at 5, and when grown in Cali- 
fornia it is no more resistant than is calif ornica when used as a rootstock 
for producing vines, and the resistance of the latter is ranked at 4. Yet 
the labrusca derivatives are extensively grown in the northeastern states 
and in other northern temperate regions. This is explained by the fact 
that the phylloxera itself, does not thrive below a certain minimum 
temperature. Thus we find that resistance to phylloxera in the species 
of Vitis varies all the way from zero in vinifera to practically absolute 
resistance in rupestris, rotundifolia and certain hybrids and that the 
existence of highly resistant forms which are also suitable for vineyard 
culture has made possible the preservation of an important agricultural 
industry. 

The question of the nature of the cause of resistance to phylloxera 
has received rather wide attention among investigators, but it has not 



BREEDING DISEASE-RESISTANT PLANTS 405 

yet been definitely answered. According to Ravaz, a chemist has 
thought to measure resistance by the amount of resinous principles in 
the roots; a physician by the relative duration of the roots; an anatomist 
by the relative thickness of the medullary rays; but all these explanations 
have failed to withstand investigation. Foex states that resistance was 
first thought to be due to great vigor, large root development and ease 
of production of new roots but that this was insufficient since some 
vines of small vigor, like Vitis monticola, are resistant while others of 
great vigor are susceptible. Foex, himself, traces a relation between 
the thickness and succulence of the bark of the root and susceptibility. 
There is also a theory, which originated in Italy, that resistance is due to 
acidity of the sap and the degree of acidity is highest in seedling plants 
and in clones which have recently come from seedlings, the acidity 
decreasing with the age of the variety. But this is contradicted by the 
fact that vinifera seedlings are quite as susceptible as their parents. 
Variability in resistance of several varieties of grape when grown in 
different infested localities is aecepted by Grassi as evidence of the 
existence of "benignant" and "malignant" races of phylloxera. But 
this does not explain the liigh resistance or immunity of some 
American species. Having in mind the fact that the phylloxera sucks 
its nourishment from the leaf or root by inserting its prolonged rostrum 
into the living tissue, it seems most probable that resistance is to be 
explained as absence of response to a specific stimulus. The many 
remarkable instances of hypertrophy in vegetative tissues due to wounds 
inflicted by insects can be explained satisfactorily only by assuming that 
the insect injects something into the wound which causes abnormal 
functioning of the affected parts. If this occurs in the case of phylloxera 
then resistance consists in failure of the wounded tissue to respond to 
the foreign element injected by the insect. Such failure of response 
might be due either to the absence of a particular substance which reacts 
so as to stimulate growth or to the presence of a specific anti-body which 
counteracts the effect of the insect's poison. The latter seems the more 
probable condition in view of what is now known concerning immunity 
in general. The complete susceptibihty of V. vinifera would then be due 
to absence of the anti-body. But the absolute resistance or complete 
immunity of V. rotundifolia may be caused by the presence of a sub- 
stance which is actually repellent to the insect itself. At any rate, the 
fact that we are dealing here with distinct natural species makes it rea- 
sonably certain that resistance and susceptibility to phylloxera infesta- 
tion are somatic expressions of genotypic diversity. 

Another important case of variation in disease resistance among 
species of the same genus is found in the relation of various chestnuts 
to the very destructive bark disease caused by the fungus, Endothia 



406 



GENETICS IN RELATION TO AGRICULTURE 



parasitica. The parasite is a native of eastern Asia where it is parasitic 
upon native species of chestnut, to which it appears to do relatively 
little harm. In other words these species are highly resistant to the 
parasite. However, when the fungus was introduced into America, pre- 
sumably in nursery stock some 25 years ago, it found in our native species, 
Castanea americana, a very susceptible host (Fig. 165). The parasite has 
already caused the destruction of the American species throughout the 
northern Appalachian region and is strongly threatening its complete 
extinction as a timber tree. Investigations have determined that the Euro- 




FiG. 165. — An advanced stage of the chestnut bark disease, caused by Endothia 
parasitica, a virulent pathogenic fungus from China. {From the Journal of Heredity.) 

pean chestnut is also susceptible to the attacks of this fungus, so that the 
future existence of this species is also jeopardized. The American chest- 
nut is one of our most valuable forest trees and its destruction will entail 
an enormous loss. A very promising Chinese species is known pro- 
visionally as C. mollissima. While it is scarcely a timber tree as compared 
with our native species, yet it may thrive in our climate. As the nuts 
are of good quality and the tree has shown marked resistance to experi- 
mental inoculations on plants already established in this country it is 
hoped that it will prove to be a successful substitute for the vanishing 
American species. Even the culture of the American species for com- 
mercial nut production in western North America will be constantly 
threatened. Hence it is fortunate that bTeeding experiments with the 



BREEDING DISEASE-RESISTANT PLANTS 407 

chestnuts have ah-cady })ceii under way some 20 years. To these we 
shall refer again. 

A bacterial organism which finds similar wide diversity in the resis- 
tance of possible hosts is the fire-blight pathogene, Bacillus amylovorus. 
Being indigenous in eastern North America, this organism must have 
maintained itself on the native species of apples and related genera 
l)revious to the introduction of European apples, pears and quinces since 
it cannot survive long even in the dead tissues of the host. The dis- 
ease is spread naturally by insects that visit infected plants; it may 
also be carried on pruning tools. Fire-blight is the most widely de- 
structive of all pomaceous fruit diseases; but the pathogene manifests 
different degrees of virulence in different species. Its most susceptible 
hosts are the commercial varieties of the pear, which are all derivatives 
of the European species, Pyrus communis. In several regions naturally 
well adapted for pear growing the culture of this fruit has been abandoned 
on account of the destructiveness of pear-blight. Even the more resistant 
varieties of communis as well as certain hybrids between communis and 
other species, such as the Kieffer, a supposedly resistant pear, have all 
proven to be susceptible to the disease when grown in the humid climate 
of the southern States. Therefore in discussing the problem of blight- 
resistance in pears it must be remembered that the pathogene itself is 
very susceptible to environmental conditions and that a particular host 
which is known to be resistant under one set of conditions will not neces- 
sarily prove to be generally resistant. Hence the breeding of blight- 
resistant pears should be carried on in a region ideal for pear culture in 
every respect except that it is ideal for the fire-blight organism also. 
Such conditions exist in southern Oregon where Reimer has made a 
complete collection of the known species of pears and has conducted 
scientific tests of their resistance to blight by means of inoculations with 
pure cultures of the bacillus. The results to date indicate that the fol- 
lowing species are highly resistant: Pyrus sinensis, P. ovoidea and P. 
variolosa. Under P. sinensis he finds there are several distinct species 
which will be classified after they have fruited, but they are all resistant. 
The birch-leaved pear, P. betulifolia, which is used as a stock in China, 
proved susceptible when the inoculations were made on 1- and 2-year 
old trees. But it is probable that older trees will show greater resistance 
and the same may be said of the 16 other species in which inoculation 
established the disease and which might be considered as susceptible 
varieties. However, varying degrees of susceptibiUty were exhibited by 
these species. Hansen reports the birch-leaved pear as quite resistant 
to blight in South Dakota where it has grown for over 20 years. 

We have now considered one plant disease which may be considered 
typical of each of the three great classes of disease-causing organisms and 



408 GENETICS IN RELATION TO AGRICULTURE 

in each we find the same diversity among natural species as regards 
disease resistance. It is unnecessary to multiply instances further. In 
all likelihood the resistance of the Chinese chestnut to Endothia parasitica 
and of the Chinese Sand Pear to the fire-blight bacillus is due to some 
specific quality of the protoplasm probably something in the nature of 
an antitoxin. That this quality is heritable will be seen from the results 
of hybridization experiments. 

Breeding Disease -resistant Varieties by Hybridization. — Allusion 
was made in Chapter XX to the fact that first generation maize hybrids 
are often more drouth resistant than either parent. Presumably this 
is merely one manifestation of heterosis. Hybridization is a very im- 
portant means, however, for the production of improved varieties which 
are better adapted to specific adverse elements of the environment. 
Witness the important results already secured in the production of cold- 
resistaiit varieties of fruits, grains and forage plants, by Hansen, Patten 
and Saunders and at the U. S. Agricultural Experiment Stations in 
Alaska. 

At one stage in the anti-phylloxera campaign in France and California 
viticulturists held definitely to the ideal of securing through hybridi- 
zation "a vine that, while resisting the phylloxera, the two mildews, 
the black rot, etc. (all of which diseases are natives, and which the 
American vines resist more or less well), will give without grafting a grape 
that has size and the quantity and quality of the Vitis vinifera." With 
this object in mind many crosses were made but they have produced no 
hybrids between vinifera and American species that can be substituted 
for the choice vinifera varieties. It, therefore, became necessary to util- 
ize resistant species and hybrids as stocks on which to graft the pro- 
ducing varieties. However, it is still possible that, by growing large 
numbers of F2 and Fs seedhngs from some of the most promising Fi 
hybrids, the dream of the viticulturist might be reahzed. It seems that 
no grape breeders have carried out extensive tests of hybrids beyond the 
first generation from the cross. This is not strange inasmuch as grape 
breeding for phylloxera resistance was at its height during the latter part 
of the 19th century and before the importance of testing for several 
generations after a cross was generally appreciated. That phylloxera re- 
sistance and susceptibility are conditioned by specific genotypic elements 
is evidenced by the results of Rasmuson who tested F2 seedlings from 
several crosses between certain American species and between American 
species and V. vinifera, as well as crosses between different varieties of 
vinifera. The latter, he reports, yielded only susceptible offspring while 
the crosses between different American species gave both resistant 
and susceptible offspring, the latter being in the minority. Resistance 
appeared to be dominant and susceptibility recessive in the progeny of 



BREEDING DISEASE-RESISTANT PLANTS 



409 



crosses between American species and vinifera. The data are not given 
but he believes the observed numbers of resistant and susceptible vines 
favor the assumption of two factors that condition immunity when either 
is present alone or when both are present together. 







Fig. 166. — a, Sandcherry, Primus besseyi; B, Wyant plum, P. americana; C, D, F2 hybrids 
from Sandcherry X Wyant. {After Beach and Maney, Iowa A. E. S.) 

Resistance to aphis in the stone fruits is thought to be a heritable 
character from the result of crosses made at the Iowa Experiment Station 
(Fig. 166). The data permit no reliable conclusions regarding the geno- 



410 



GENETICS IN RELATION TO AGRICULTURE 



typic relation of aphis resistance and susceptibility in these plants, but 
the indications are that these characters are conditioned by a single 
factor difference. Another interesting case of the inheritance of resist- 
ance to aphis was observed by Gernert in Fx hybrids between teosinte and 
corn. Both the corn root-aphis, Aphis maidiradicis, and the corn plant- 
aphis, A. maidis, were involved, and both the teosinte and the hybrids 
were completely resistant while the corn was badly infested. The desira- 
bility of securing aphis resistant varieties of maize will be apparent when 
it is realized that most of the corn growing regions of North America are 
infested with these insects and that the loss in reduction of yield caused 
by them is enormous. 

The work of Van Fleet on hybridizing various species of chestnuts 
was begun 10 years before the terrible bark disease had worked havoc 




Fig. 167. — In the center is a nut produced by a cross between the American bush 
chinquapin, Castanea -pumila, (right), and the Japanese chestnut, C. crenata, (left). Al- 
though intermediate in size the hybrid nut is disease resistant and of good quality. (From 
The Journal of Heredity.) 



with the chestnut trees near New York City, which is the oldest known 
center of infection. Hence many crosses were made with either the 
American or European chestnut as one parent, but in 1907 these were all 
destroyed by the Endothia. Fortunately however numerous controlled 
polhnations were made on the bush or Virginia chinquapin, Castanea 
pumila, using pollen of a Japanese species, C. crenata, as well as other 
Asiatic chestnuts. It is asserted that the Asiatic species and the chin- 
quapin-Asiatic hybrids are highly resistant, because few have shown any 
appearance of infection although surrounded by diseased trees, and that 
even when infection takes place the injury is quite local in character. 
Van Fleet adds that second generation seedlings of chinquapin-crewa^a 
crosses show no disease although constantly exposed to infection (Fig. 
167). Thus a beginning has been made in what promises to be an import- 
ant branch of nut breeding, and the orchard production of commercial 
chestnuts has been insured against future encroachments by a deadly 
disease through the timely efforts of a zealous and far-sighted plant 
breeder. , ' 



BREEDING DISEASE-RESISTANT PLANTS 411 

In an attempt to breed blight-resistant pears of horticultural value 
Hansen has produced and distributed for trial thirty-nine first generation 
hybrids between various commercial varieties and either the Chinese 
Sand Pear or the Birch-leaved Pear. Should these hybrids prove to be 
unsuitable as commercial varieties they may be used as foundation stock 
in further efforts to produce a hardy, blight-resistant variety. Although 
the Kieffer and the Le Conte are presumably Fi hybrids between sinensis 
and communis, they have not been used by Hansen because they are not 
hardy in the north. For lower latitudes however these two partially 
resistant varieties should be utilized not only by raising seedlings from 
them but also by an extensive series of crosses especially with other 
partially resistant communis derivatives of high quality such as the 
Seckel. The work of Reimer and of Hansen indicates that perfectly 
resistant stocks may be developed which are adapted for each important 
pear-growing region. If to this achievement may be added the creation 
of fairly resistant varieties of really excellent quality, the worst diffi- 
culties in pear production will be removed and the world's supply of 
this delicious fruit will be practically assured. 

Creating Rust-resistant Commercial Wheat by Crossbreeding. — The 
grain rusts are the most important of all fungous plant diseases. The 
annual losses they entail for the grain crops of the world must be estimated 
in the hundreds of millions of dollars. Although prevention of wheat 
rust to some extent is now possible by giving careful attention to the 
water and soil relations of the wheat plant and by early seeding or the 
planting of early varieties which sometimes escape attacks by rust, yet 
these diseases still remain a serious menace to the maximum production 
of wheat. Hence, the creation of rust-resistant varieties has become a 
very important problem. The diversity among varieties of wheat as 
regards resistance and susceptibility to rust fungi was recognized by 
Knight in 1815 and the desirability of creating new varieties which should 
be resistant to rust as well as highly productive and of good milling 
quality was fully realized by such breeders as Pringle, Blount and Farrer. 
Although they were not famihar with the Mendelian principles of seg- 
regation and recombination of characters, these breeders of wheat, a 
self-fertilized annual crop plant, were naturally led to persist in their 
efforts beyond the Fi generation. The work of Farrer expecially was 
thorough and reliable. He found that he could not secure absolute 
resistance to the black stem-rust, Puccinia graminis Pers., combined with 
good milling quality in his wheat crosses even when rigorously selected 
in the F2 or "wild" generation as he called it. Most of the soft bread 
wheats are very susceptible to rust and, when crossed with the resistant 
durums, poulards and spelts, they give rise to strains which are either 
poor bread wheats or are rust susceptible. Biffin discovered in 1903 



412 GENETICS IN RELATION TO AGRICULTURE 

that resistance to the yellow rust, Puccinia glumarum Eriks. & Henn., 
in his cross between RiVet, a slightly susceptible wheat and Red King, 
a very susceptible variety, was recessive in the Fi generation but appeared 
in approximately one-fourth of his F2 population. Tests of later genera- 
tions proved that this character bred true. Eriksson tested Biffin's 
work and found only slight variations in the F2 ratio and in the intensity 
of the resistance. However, it appears that resistance of the wheat 
plant to other species of rust fungi may be inherited as a dominant char- 
acter. Vavilov reports that he crossed Persian wheat, Triticum vulgare 
var. fuliguosum Al., which alone out of 540 varieties was immune to 
mildew, Erisiphe graminis DC, but which was susceptible to brown rust, 
Puccinia triticina Eriks., with other varieties of common bread wheats 
and secured Fi hybrids which were immune to both diseases. Thus it 
is clear that the inheritance of rust resistance is dependent upon the 
specific relation existing between the parasite and the host. 

The practical aspects of breeding rust-resistant cereals is greatly 
complicated by the fact that resistance in a single variety of wheat, for 
example, is likely to vary geographically. While this is due in part to the 
responsiveness of the wheat plant to radical changes in environment, it 
is probably more often due to physiological variations in the rust fungi. 
The virility of a given parasite appears to vary not only with the host 
but with the geographical location. A striking example of this was 
observed by .Mackie in the behavior of Kubanka, a durum wheat of 
Russian origin. Although this wheat is remarkably rust resistant in the 
northern Great Plains region, yet when grown on the west coast of 
Mexico it succumbed completely to the stem rust {Puccinia graminis 
var. tritici) which it had resisted successfully in the Dakotas. The ex- 
planation of this failure of a supposedly resistant wheat is found in the 
existence of local physiological races of the species P. graminis. Thus 
Freeman and Johnson found P. graminis var. tritici, which is supposedly 
confined to wheat, attacking barley and rye as well. The same results 
were obtained with oat stem rust, P. graminis var. avence, which readily 
attacked barley but was less virulent on wheat and rye. The stem rust of 
barley was found to be most readily transferred to the other cereals. 
In addition to the barberry numerous wild grasses serve as hosts of 
the stem rusts which fact still further complicates the problem of breeding 
for rust resistance. Starkman and Piemeisel have investigated the rusts 
of about 35 species of grasses and have found six distinct biologic forms 
of this species of rust, one of which came from an isolated area. Among 
other important discoveries, they found that more than one biologic 
form may occur on the same host in nature, sometimes even on the same 
plant; that these biologic forms can be distinguished from each other 
morphologically as well as parasitically; that different strains of the same 



BREEDING DISEASE-RESISTANT PLANTS 413 

biologic form sometimes differ in degree of virulence on the same host; 
and that all gradations in susceptibility occur among the hosts, from 
complete immunity to complete susceptibility to various biologic forms. 

Finally, it must be remembered that but little is yet known about 
the nature of rust resistance. That it is in no wise dependent upon 
morphological characters appears to be well established. Carleton has 
pointed out that biochemical investigations are needed in connection with 
this problem. The recent investigations of Wagner on hydrogen ion 
concentration and natural immunity in plants representing four genera 
including the potato resulted in the conclusion that the variation in 
hydrogen ion concentration in plant tissues is a phenomenon of reaction 
to the injection of pathogenic bacteria. Also that the course and end 
results are related to the susceptibility of the plant in question and to the 
character of the disease as acute or chronic. An investigation now in 
progress at the University of California (by W. W. IMackie) seems to 
indicate that there is positive correlation between degree of acidity as 
indicated by the concentration of hydrogen ions and degree of resistance 
to P. graminis in wheat. A similar investigation of the species of 
Bromus in relation to the physiological races of the Corn and Grass 
Mildew, Erysiphe graminis DC, as reported by Salmon would be highly 
desirable. Whatever the nature of the resistant quality may be, there 
is no question regarding its heritability. But in view of the complicated 
nature of the problem which we have briefly outlined it would appear to 
be inevitable that the utilization of resistant varieties of wheat must be 
confined to limited areas in which adequate tests have proven their 
adaptability. 

Inheritance of Disease Resistance in Other Plants. — The conclusions 
we have reached in respect to rust resistance hold good in a general way 
for other parasitic plant diseases. In addition to the typical cases 
already described brief reference may be made to other notable examples 
of the successful creation of disease resistant varieties by hybridization 
and subsequent selection. The next case, however, will be considered 
somewhat in detail because it serves as an excellent model in method of 
procedure. The ravages of a group of wilt diseases caused by closely 
related fungi of the genus Fusarium have been checked through the 
successful efforts of the United States Department of Agriculture. As 
reported by Orton these are the cotton wilt, Fusarium vasinfectum Atk., 
the cowpea wilt, F. tracheiphilum Erw. Sm., and the watermelon wilt, 
F. niveum Erw. Sm. It is clear that these fungi possess a high degree of 
adaptation to the parasitic mode of existence. Also that, while the cause 
of resistance-in certain varieties of the host species is not fully established, 
yet the resistance itself is a physiological quality. No constant mor- 
phological differences have been detected between immune and suscepti- 



414 



GENETICS IN RELATION TO AGRICULTURE 



ble plants; neither are there observable differences in time of germination, 
rate of development or period of maturity. Furthermore, the resistance 
is specific; varieties that resist the wilt may be susceptible to bacterial 
blight and vice versa. 

That wilt resistance is a heritable character was strikingly dem- 
onstrated by Orton's creation of a wilt-resistant edible watermelon, 
Citrullus vulgaris. All watermelons appear to be very susceptible to 




Fig. 168.— Parent 



and product <if tliird generation offspring: 
hybrid. {After Orton.) 



watermelon-citron 



the disease. Extended tests in 1900 and 1901 failed to show any basis 
for selection among the 120 or more varieties tested. Recourse was had, 
however, to an inedible form of Citrullus vulgaris known as citron or stock 
melon which was immune to wilt. From a cross between this citron and 
the Eden variety of watermelon Fi hybrids of "wonderful vigor and 
productiveness" were raised. The fruits were intermediate in character, 
having the oval form and stripes of the watermelon and the hard flesh of 
the citron. The F2 population was extremely variable in every respect, 



BREEDING DISEASE-RESISTANT PLANTS 415 

the various citron characters appearing to be dominant in the majority 
of plants. From among 3000 or 4000 plants ten fruits were selected on 
the basis of resistance and quality and the seeds were planted the follow- 
ing year, 1904, in isolated, infected plots. Of these ten plots two were 
found to bear melons of uniform appearance and quality one of which 
resembled the Eden parent. These were sesquihybrids from the Fi 
pollinated by Eden. Again all the best melons were selected and planted 
separately the following year and further variations were found. After 
five more years of selection a variety was obtained whi(!h had great uni- 
formity and disease-resistance while the fruit had a thin, tough rind which 
enables it to endure long railway shipments. The flesh is so juicy 
that the melons are heavier than Eden melons of the same size; the 
quality and flavor are good although not equal to the finest (Fig. 168). 
These qualities have been preserved and resistance maintained at a dis- 
tance of 740 miles from the place of origin, but on the Pacific Coast the 
resistance was not maintained. 

This failure of the supposedly resistant variety when grown in a far 
distant locality is not strange when we remember that the wilt fungi are 
highly specialized in their adaptation to hosts. According to Orton 
Fusarium niveum attacks no other living plant than the watermelon and 
"in this respect, coupled with their close morphological resemblance and 
their common geographical distribution, they seem to be analogous to the 
biological strains of Puccinia and Erisyphe." In combating all such 
diseases the importance of developing locally adapted varieties must not 
be overlooked. 

The specific nature and heritability of disease resistance is also evi- 
denced by the results of numerous other experiments among which may 
be cited the following. In the tomato wilt resistance was found by 
Norton to be recessive to susceptibility and varieties of Fusarium-resis- 
tant tomatoes from Tennessee were found to be susceptible in Maryland, 
Stuckey found that cherry, pear and currant tomatoes were immune to 
the blossom-end rot, a functional disease and, when crossed with com- 
mercial varieties, they transmitted resistance as a dominant character. 
Resistance to leaf blight in the cantaloupe was found by Blinn to be 
inherited as a dominant character. Jesse B. Norton when breeding for 
resistant varieties in combating the asparagus rust, Puccinia asparagi DC, 
found resistance dominant in all the Fi offspring in his crosses between 
the female plants of the rust-susceptible American varieties and a rust- 
resistant European asparagus. The resistance was somewhat variable 
but was fixed by selection in succeeding generations. These few cases, 
taken almost at random, together with the typical illustrations already 
discussed, amply justify the recommendation that the breeding of disease- 
resistant varieties of economic plants by hybridization and subsequent 



416 GENETICS IN RELATION TO AGRICULTURE 

selection should receive more attention from plant pathologists and 
horticulturists in the future. 

Breeding Disease -resistant Plants by Selection. — Selection alone is a 
powerful means of improving plants with respect to disease resistance 
when used either in variety tests or in the improvement of a single variety. 
The testing of varieties for disease resistance is an exceedingly important 
service which can be done most satisfactorily by experiment stations and 
commercial seedsmen in connection with their routine work. However, 
the geographical variability in many parasitic organisms and the impor- 
tance of local adaptation of varieties in many economic plants make it 
imperative that each important agricultural region should have its own 
station for variety testing. 

The diversity between varieties in respect to disease "resistance" is 
sometimes due to morphological or anatomical peculiarities which prevent 
the invasions of parasites. For example, pubescence or waxy excretions 
on the surface sometimes prevent disease; the number of stomata or the 
arrangement of cells beneath them may condition fungus infection. 
Also some varieties escape certain diseases by virtue of their seasonal 
adaptation or because of precocity. Thus certain grains are less troubled 
with smut than others because they germinate more quickly. A differ- 
ence of 2 days in time required for germination may be the deciding 
factor in smut infection. Certain varieties of potatoes are able to form 
a corky layer in about 6 hours after being cut while others require 3 
or 4 days. Bacteria require from 12 to 24 hours to commence 
putrefaction through enzyme action. In addition to these and 
many other varietal differences there is always the possibility of real 
immunity which is due to some specific physiological character of the 
variety. A probable instance of considerable importance is the immunity 
of milo to the smut fungus which is infectious to all other sorghums. 

Although there are numerous valuable reports on the disease relations 
of certain varieties of our important crop plants, much remains to be done 
in the way of both extensive and intensive testing. The following 
citations merely illustrate the kind of information that is now available. 
Recent observations at the Kansas Station on 119varietiesof winter wheat 
showed infection with orange leaf rust, Puccinia rubigo-vera tritici 
Carleton, varying from 5 to 90 per cent. According to Orton there are 
varieties of the potato which are partially resistant to late blight and 
probably also to scab, a matter which has received considerable attention 
abroad but very little in this country, although there are undoubtedly 
great possibihties in this work. Orton's success in producing wilt resist- 
ant varieties of cowpea was made possible by the discovery of one immune 
variety, the Iron, which was apparently of chance origin. In future 
breeding work much time and effort might be saved if agronomists and 



BREEDING DISEASE-RESIST A NT PLANTS 



417 



horticulturists would insist on accuracy with respect to the specific 
diseases observed in all records of disease resistance or immunity. 

The selection of disease-resistant strains is a simple but effective 
method of improving commercial varieties of many crops. Of course the 
eflEicacy of this method in autogamous species is dependent upon the occur- 
rence of mutations or natural hybrids. This is the reason why selection 
for rust resistance within a variety of wheat is usually wasted effort. 
But in the majority of crop plants there is more or less crossing and con- 
sequently more or less likelihood of picking desirable combinations of 




Fig. 169. — Breeding field of upland cotton planted with progeny rows each from the seed 
of an individual plant. Note difference in resistance to wilt disease. (After Orton.) 

disease-resistant factors. As a result of efforts to find a wilt-resistant 
Sea Island Cotton several strains were obtained by planters and by the 
U. S. Department of Agriculture, all of which are resistant enough to 
grow on the worst infected land. Upland cottons are even more sus- 
ceptible to wilt, but varieties are now grown which are very resistant to 
wilt and of excellent productiveness (Fig. 169). All of these improved 
varieties of cotton have been secured by continuous selection beginning 
with resistant individuals. As a result of his experience in improving 
varieties of flax for wilt resistance, Bolley has emphasized the importance 
of continually subjecting the select strains to conditions favorable to the 
disease but otherwise optimum for the plant. Spragg isolated a strain 
of alfalfa resistant to leaf-spot by selecting from a few resistant plants 

27 



418 GENETICS IN RELATION TO AGRICULTURE 

which were discovered in the 1913 nursery, the result presumably of a 
mutation. Johnson reports the results of 2 years' experiments on 
the relative resistance of selected strains of tobacco to the root rot, 
caused by Thielavia basicola (B & Br.) Zopf. A strain of White Burley 
tobacco has been developed which possesses a high degree of resistance. 
The Wisconsin Station has successfully combatted the "yellows" 
and the black rot of the cabbage by selection. Both are very destructive 
diseases, the first being caused by a fungus and the second by a bacterium. 
Starting with a commercial variety that is resistant or immune to the 
"yellows" a strain which is also highly resistant to black rot has been 
developed. "A stand of from 95 to 99 per cent, of the rot-proof type 
was obtained on some fields where both the imported Danish and Puget 
Sound seed failed to give more than 15 to 20 per cent, of a crop." The 
practical value of such simple selection as the elimination or roguing of 
all diseased or weakly plants, an ancient practice, must not be over- 
looked. As was pointed out in Chapter XXIII, much of the value of hill 
selection in potatoes doubtless lies in the elimination of diseased tubers. 
Finally it must be remembered that some diseases or "off-type" 
states in plants are caused by environmental conditions to overcome 
which it would probably be impossible to select resistant strains. A case 
in point is Yellow-berry in wheat which is described as the appearance of 
yellow or white, mealy or half-mealy, or spotted grains, otherwise without 
apparent blemish. Its occurrence is believed by Headden to indicate 
"that potassium is present in excess of what is necessary to form a ratio 
to the available nitrogen present advantageous to the formation of a 
hard, flinty grain." He concludes that it is entirely within the control 
of the grower through employment of proper cultural methods. How- 
ever, the universal occurrence of Yellow-berry on the Pacific Coast 
points to something more profound than the potash-nitrogen ratio as the 
determining cause. If this general factor is climatic, wheat breeding 
projects on the Pacific Coast should be organized with reference to it in 
the fundamental tests of commercial varieties and of the various species 
and sub-species of wheat for the purpose of planning more promising 
hybridization experiments. Attempts at improvement of existing varie- 
ties for resistance to Yellow-berry by selection would appear unwarranted. 
Especially is this the case if the disease can be counteracted by cultural 
methods. The need for resistant varieties is also less imperative in the 
case of many destructive diseases for which methods of control have 
been successfully devised. Yet it is obvious that an enormous saving 
to agriculture would come from the production of such varieties. There 
is a vast field here for the combined efforts of the pathologist and the 
geneticist. 



CHAPTER XXV 
PLANT -BREEDING METHODS 

In no phase of agriculture is there greater need of scientific planning 
with reference to economy of time and resources as well as efficiency of 
method than in plant breeding. In annual species the individual plant is 
of small intrinsic value as compared with a domestic animal, and gen- 
erations follow each other in rapid succession. These facts tend to en- 
courage methods that are wasteful or inefficient, or at any rate methods 
that fail to accomplish all that might be accomplished in a given time. 
In work with perennial species the need of a scientifically planned system 
of breeding is even more urgent because of the greater intrinsic value of 
the individual plant and the longer time required to obtain results. 

Pedigree Culture Methods. — The pedigree culture was first used in 
a systematic manner by Vilmorin in breeding wheat. Later it was 
adopted by Hays of Minnesota and by Nilsson in Sweden. The essen- 
tial feature of the pedigree culture consists in rearing successive gen- 
erations of organisms under such conditions that the ancestry of each 
individual is known. Its purpose is to insure absolutely accurate knowl- 
edge of ancestry. To attain to this ideal many precautions are necessary, 
and some sources of error, due principally to accident, cannot be entirely 
eliminated. However the same accident is not liable to happen twice to 
the same culture and when an accident does occur sometimes the culture 
can be repeated. Any material whose pedigree is in doubt should be 
eliminated at once or, if rare or valuable, relegated to the class of unknown 
until its own behavior in breeding indicates its genotypic nature. 

Seedage methods in pedigree culture work with plants are very im- 
portant. Handling seeds and seedlings requires quite as much care as 
does castration and pollination especially with very small seeds and this 
work should be done by the person in charge if possible. However, 
the work of transplanting and field planting can usually be performed by 
a person especially adapted and trained for it. The work of taking 
notes, collecting seed and recording data, on the other hand, should 
be done by the investigator or breeder. The original label should be 
collected with the seed and preserved until the permanent records are 
made; then it should be carefully copied and compared with the written 
notes before being discarded. As a rule it is well to count the seeds in 
each bag inasmuch as hybrid seed may prove to be partially sterile or 
may exhibit delayed germination or other abnormalities. 

419 



420 GENETICS IN RELATION TO AGRICULTURE 

Planting pedigree cultures involves some of the most difficult prob- 
lems especially in handling small seeds. Larger seeds such as wheat, 
corn, peas and beans are handled satisfactorily by planting a single seed 
in a paper planting pot containing thoroughly sifted soil. If more than 
one seedling of the species planted should appear in a single pot, it should 
be discarded. These individual plants should then be set out at equi- 
distant points in the rows and each row labeled. Also at the time of 
planting a memorandum should be made of the plan of the plot with the 
position or number and contents of each row as a safeguard in case of loss 
of labels. Small seeds like tobacco, petunia, primula and even those like 
the strawberry should be planted in sterilized soil in order that any seeds 
of the same species that happen to be in the soil will be destroyed. The 
most satisfactory method of sterilizing soil for seedage is to steam it under 
pressure. The soil is placed in the clean earthenware pots or seed pans, 
in which the seeds are to be sown, and these go directly into the auto- 
clave, where they should remain under pressure of 15 pounds to the square 
inch for at least 1 hour. Where gas is not available for heating the 
autoclave an alcohol blast lamp may be used. If it is not convenient to 
prepare the sterilized soil fresh each day as needed, the pots not intended 
for immediate use should be covered before being sterilized with a piece of 
fine-meshed fabric which is securely tied below the flange of the pot. This 
covering is left in place until the pot is used . The manipulator should have 
clean hands, clothes and utensils and should handle only one lot of seed 
at a time using due precaution between each lot to avoid mixing. Each 
lot should be labeled as soon as sown. The seed pots or pans are then 
moistened by setting them in a vessel containing water, care being 
taken not to let the water overflow into the seed vessels. All subsequent 
watering should be by means of sub-irrigation rather than surface 
watering when dealing with small seeds, or if this is not practicable the 
water used in sprinkling should be filtered or strained. When ready to 
prick out, the little seedlings may be transplanted into unsterilized soil 
provided they are set at uniform distances so that any foreign seedlings 
that appear later can be distinguished with certainty. This is easily 
accomplished by selecting a board the same size as the flat and ruling it 
into squares, then driving tenpenny nails through the corners of the 
squares. With this tool the holes are made for a whole flat of seedlings 
at once and they are uniformly spaced. If the seedlings are minute it is 
a wise precaution to mark each as it is pricked out by sticking a tooth 
pick into the soil close to it. 

Protection of cultures also involves problems which are highly impor- 
tant in pedigree plant breeding but difficult to discuss without entering 
into considerable detail. The danger may appear in the form of curious 
or ignorant persons who do not understand the importance of keeping 



PLANT-BREEDING METHODS 



421 



hands off. Or a strong wind may succeed in forcing into the greenhouse 
seeds of the same species as the one undergoing investigation. Or pests 




iiG. 170. — Birdproof cereal breeding garden at the University of California. 



/ / 





Fi... 



I'' iliL'iee cultures of Bursa in greenhouse of the Dcitartnient of Botany, Prince- 
ton University. {Photo from G. H. SItull.) 



of various sorts may appear — damping off, insect pests, snails, slugs, mice, 
and in the breeding garden birds, gophers, moles, rabbits, etc. In this 
connection it may be sufficient to say that eternal vigilance is the price 



422 GENETICS IN RELATION TO AGRICULTURE 

of success. Anyone with ordinary ingenuity will usually be able to 
provide the necessary protection. The important thing is to realize 
its need in time to prevent loss or contamination of cultures. If the 
breeding garden is located in or near cities the English sparrow will 
work havoc with developing seeds, especially of cereals. This menace 
can be completely overcome only by enclosing the threatened cultures 
with something that will keep out the birds and at the same time cut off 
the minimum amount of light. We have found 1-inch mesh poultry 
netting satisfactory (see Fig. 170). Many plants used in genetic 
investigations can be handled most satisfactorily in the greenhouse. 
Fig. 171 shows a greenhouse filled with pedigree cultures of a single species. 
A systematic method of recording and preserving data is a sine qua 
non for the pedigree culture. It is absolutely unsafe to trust to memory 
if any degree of accuracy is to be attained. For work on a small scale 
a serial number (using Arabic numerals) for each culture is satisfactory. 
This number then becomes the permanent designation of the given 
culture, each plant in the row receiving a subscript number. Thus 
plant No. 5 of culture No. 3 would be designated as 3P5. ' If this plant is 
selected for further testing with self-pollinated seed, its progeny in the 
next generation will be labeled ZFi P5 Pi, 3F1P5P2 and so on down the 
row of plants. However, this method is rather cumbersome and for 
work upon a large scale the "annual-note-book-page" method first 
described by Shull is much more satisfactory. In this system each 
culture of a given year is numbered chronologically receiving the number 
of the page in the note book for that year on which it happens to be 
recorded. The label bears this number preceded by the distinctive 
numerals of that year. Thus the particular culture recorded on page 
1 of the 1918 note book will be labeled 181 or 18.1. The use of the 
decimal point is a convenience especially if one is working at an in- 
stitution where serial numbers are in use in another department. In 
addition to the annual note book a set of permanent index cards should 
be arranged each year, including of course only those actually grown in a 
given year. By writing the current year number in one corner and the 
corresponding number for the preceding year in the other corner one has 
a convenient system for securing the complete pedigree of a given cul- 
ture. To complete this method some designation is necessary for the 
individual plants selected in any year. This may be a number in paren- 
theses, a subscript, a letter, or, where plants are set at equal distances 
from a given base line and each plant is thus numbered automatically, 
the letter P with subscript is satisfactory. Whatever the individual 
designation may be, it becomes the name of the particular plant for the 
remainder of its existence but its progeny will receive a new number 
when the seed is sown. 



PLANT-BREEDING METHODS 



423 



A system of labeling and recording that will be at once concise and 
definitely descriptive of the individuals and the nature of the matings 
has obvious advantages. Pearl has devised a system which is especially 
useful in crossbreeding experiments and in work with self-sterile plants ; it 
can be adapted for any material. By the use of letters to denote indi- 
viduals or types of individuals that are brothers and sisters and numbers 
to denote types of matings a perfectly general set of terms is provided 



R 



F 



F, 




A B 



F C 



Types of Mating in F2 



Fi indi- 
viduals 
mated 


Number 

of 
mating 


F2 indi- 
viduals 
mated 


Number 

of 
mating 


F2 indi- 
viduals 
mated 


Number 

of 
mating 


Fa indi- 
viduals 
mated 


Number 

of 
mating 


AXX 


10 


B XZ' 


46 


C XF 


51 


EXE 


19 


AXY 


12 


B XB 


13 


C XG 


53 


E XF 


45 


AXZ 


40 


B XC 


37 


DXX 


22 


E XG 


47 


A XZ' 


42 


BXD 


29 


D XY 


24 


F XX 


30 


A XA 


11 


B XE 


55 


D XZ 


52 


F XY 


32 


A X B 


33 


BXF 


57 


D XZ' 


54 


F XZ 


60 


A XC 


25 


BXG 


59 


D XD 


17 


F XZ' 


62 


A X D 


35 


C XX 


18 


D XE 


43 


F XF 


21 


AXE 


61 


C XY 


20 


D XF 


31 


F XG 


49 


AXF 


63 


C XZ 


48 


DXG 


27 


G XX 


34 


AXG 


65 


C XZ' 


50 


E XX 


26 


G XY 


36 


B XX 


14 


C XC 


15 


E XY 


28 


GXZ 


64 


BXY 


16 


C XD 


39 


E XZ 


56 


G XZ' 


66 


B XZ 


44 


C XE 


41 


E XZ' 


58 


GXG 


• 23 



Fig. 172, Table LVI. — Illustrating a system of labelling Fi and Fz individuals resulting 
from any type of mating. (Adapted from Pearl.) 

Order of precedence: female named first in every cross; X 9 X Y d' = mating (1); 
Y9 XXd' = mating (1') ; in back-crosses Zafer generation 9 X earlier gencTation d' ranks 
first, thus Z 9 X Xd = mating (2), X9 X Zd = mating (2'), etc. 



424 GENETICS IN RELATION TO AGRICULTURE 

which can be used to describe any pedigree. The scheme described 
below is essentially Pearl's system with some minor changes. In the dia- 
gram (p. 423) solid lines with circles containing numbers indicate matings 
between the individuals represented by the letters which they connect. 
Dotted lines lead from the matings to the individuals produced. The 
order of precedence in nomenclature may be varied to harmonize with 
the former practice of the investigator. Table LVI indicates every pos- 
sible combination of F2 individuals to produce F3. By priming the 
mating numbers reciprocal back crosses can be indicated and the kinds 
of individuals produced may be similarly distinguished. For example, 
following the order of precedence suggested above the diagram, Z X Y 
is mating 8 and produces E but F X ^ is mating 8' and produces E' 
while E or E' X X is mating 26 and produces, let us say, N and X X E or 
E' is mating 26' and produces N'. The order of mating in intra-fraternal 
crosses will be immaterial except in the case of sex-limited characters 
when individuals may be distinguished by subscripts. Pearl explains 
the arrangements of mating numbers in the table as follows: 

A word should be added in regard to the system by which the numbers 
have been assigned to the matings. It might at first sight appear as 
though the arrangement were an entirely haphazard one. It is not. 
On the contrary the numbers will be found to conform to the following 
general principles, which seem likely to be of aid in practical work, as 
tending to make it easy to recall from a number just what its particular 
pedigree looks like. 

1. All even numbers refer to back-cross matings. 

2. All odd numbers refer to co-fraternal or intra-generation matings 
(not back-crosses). 

3. Matings below 2 are of parental generation individuals; between 2 
and 8 inclusive are of Fi individuals; matings over 10 are of F2. individuals. 

4. Even numbers from 10 to 36 inclusive designate back-crosses of 
F2 individuals with their grandparents, or individuals of the grand- 
parental generation. 

5. Even numbers from 40 up designate back-crosses of F2 individuals 
on Fi individuals. 

6. In the case of the odd numbers from 11 up it is, in a general way, 
true that the smaller the designating number of a mating the more closely 
related to each other are the two individuals entering that mating likely 
to be.. This principle of assigning the numbers could not be so precisely 
followed as the preceding five, but still is perhaps worth a little. 

In using such a system it is of course necessary to have the basic 
table always at hand. The diagram is quickly drawn and the typewritten 
tables may be pasted in note books or both diagram and table may be 
printed on cardboard for use in breeding pens or plots. 



PLANT-BREEDING METHODS 425 

The Svalof System. — At the Swedish Institute for the Improvement of 
Field Crops, Nilsson has worked out a very complete and efficient system 
of plant breeding. Gradually, as increased appropriations of funds have 
permitted expansion, a corps of experts has been employed, each in- 
vestigator concentrating on one or two species, and thereby training 
himself to distinguish all the different forms so as to judge of the relative 
value of different combinations of characters. Furthermore a definite 
course of procedure has been developed as a result of many years of 
experience during which time marked success has been achieved in the 
inprovement of Swedish field crops. To begin with the work consisted 
mainly of variety testing and extensive effort at improvement through 
mass selection. These methods still find a place in the routine work, but 
they are of insignificant value as compared with the coordination of 
intensive methods which makes the Institute's system a model which 
institutions engaged in similar work may profitably follow. From 
Nilsson's description we find that the Svalof system may be briefly out- 
lined under three heads, viz., genotype selection, strain tests and hyhridiza- 
tion. In all this work the methods of pedigree culture are followed so 
that the original source and performance record of each form grown at 
the station or distributed for trial can be accurately stated. Genotype 
selection in all plants except the self-sterile species is accomphshed by 
the pedigree method of testing the progeny of single individuals. In 
wheat, barley, oats, peas and vetches, which were the first crops chosen 
for improvement at Svalof, this means of course the isolation of pure 
lines from the beginning, and the more recent work with rye, clover, 
forage grasses, beets, etc., has determined that allogamous species are 
composed of biotypes which are analogous to the pure lines of autogamous 
species and which can be segregated from one another by continued 
inbreeding, exactly as inbreeding in maize has been found to isolate 
biotypes. Although inbreeding must be continued for several years 
before these biotypes or strains acquire a satisfactory degree of purity 
and stabihty, yet it has been shown already according to Nilsson that 
this method can be used to bring about the same practical results as have 
been secured in wheat and other autogamous plants. In connection 
with this preliminary selection of promising forms the intensive study of 
the specialists at Svalof has made each member skilful in detecting 
different forms in the species with which he is working and in judging 
the relative value of the characters displayed. Having separated from 
the population some of the biotypes of which the ''variety" is composed, 
it next becomes necessary to subject all these strains to comparative 
tests in order that the few superior forms may be discovered and pro- 
pagated more extensively. This requires long years of careful work and 
the overcoming of certain difficulties which will be discussed later. The 



426 



GENETICS IN RELATION TO AGRICULTURE 



handling of thousands of cultures each year (in 1912 there were over 9500 
numbers in the trial plots) has required the devising of many practical 
arrangements to insure exactness and order. Each distinct strain that 
has been retained for more extensive trials is regarded as an established 
variety. Hybridization of cereals had been started at Svalof upon rather 



CLASS 



Relative developmeat of sample 
P. Itinatm, P. cocdneus. Vigna sesqcipedAHs. Vicia faba. Pole, bash. Snap, green shelled, field. good medium poor 



DATE 

Seed planted replanted plants u 



Pods fit for snaps past nse snaps fit for green shelled pjist ase shelled ripe 



PLAirr (nut resistant) Stems 

nTn^n laige erect spreading gnprodoctive unhealthy ? few many short long stoat slender. Tate pole yell light dark yellow green 

Leaves Flowers 

small large thin thick smooth rocgh light dark yellowish green red pnrple. small large, white light dark yellow red parple. Single. In pairs 
Fruiting habit 
In small large dusters well above, half hidden by foliage. Pods set poorly freely, through early late short long season nscally single 



in pairs 



in small large 



compact loose dusters 



r center octsJde of plant 



resting oo well above soil. 



PODS AS SNAPS 
short long 



(side bearing beans) 
stout slender. Ventral sntnre straight 



f stem center point 



Cross section 
r stem center point. flat oval round broad , 



Point 
e backed. From back center of pod 



short long thick slender straight curved forward backwards light dark green wax Uke 



Surface of pod 

smooth rocgh 



little much diseased? Jight dark bright dull white yellow green 



evenly self colored little much marked with ? 



Wills of pod 
lighter darker. thin thick fleshy dry brittle tough little much flber in sides much little no sfajng in back when young 



much Uitle no string when old. Pods 



long usable soon past use as strings, fade and wilf 



PODS AS GREER SHaLED 

short long slender thick 



full constricted between beans. Contain 



loose crowded in pod 



not on central line 



Bean 
few many blasts small large 



light dark green, marked wfth ? 



Green shelled beans cooked 

tough tender |nicy dry mealy poor good flavor 



DRY BEARS 
1 attractive color. Thrash out 



Shape (long section) 
easy bard. short long round oval cylindrical 



Kidney shaped, fuH at eye. Ends 



Cross section 
1 symmetrtcal, thin thick flat oval mend 



dull bright white black 



light dark red yellow brown self colored 



little much eyed dotted striped mottled with? 



VARIETY 

not distinct 



Bot. spec taken by. 





pbnts 


Var. qualitr of stock Raiket qiuUtr of ham 


BATE 


So. 
extra 


Ro. 
fail 


Not 
true 


No. pods 1st quilitv 






Soap 


Or. 

Hull. 


Drv 


S-' shdi. 


Dry 


Snap 


G.. 


Dry 











— 
















— 





















Suggestions for use of signs.— V/otds are to be erased, 
signs, wt>rds, and references to notes written in so as to leave 
as fnU desaiplloQ of stocl: as possible. Use -f to signify a 
high and - a low development of quaiity: A signifies vari- 
able; U> uniform. A connecting loop between two words 
signifies tliat stock is intermediate or variable between the 
connected words with more or less tendency toward the i]ual- 
ity indicated by the shorter arm of the loop. A cirde inclos- 
ing a sign signifies an exceptional or pheoomenal debtee. A 
large drde over a qualitv signifies that no record his been 
made of that (luality. When space is not sufficient for a 
desired record, as in cases where ? is Inserted, refer by letter 
to notes. 



Fig. 173. — Variety description blank for beans used by the Bureau of Plant Industry, 
U. S. Department of Agriculture. (Other face shown in Fig. 174.) 



an extensive scale previous to 1900 but it had not given encouraging 
results. After the announcement of MenclePs discovery this phase of 
the work was greatly enlarged, particularly with wheat and oats by 
Nilsson-Ehle. Through the application of Medelian principles new varie- 
ties of these cereals have been produced which combine general good 
qualities with cold-resistance or earliness which makes them better 



PLANT-BREEDING METHODS 



427 



adapted for more northern localities. It has been found, as would be 
expected, that in l)reeding for numerous characters at the same time the 
recombinations in F2 following hybridization of many select strains are 
so complicated as to render very illusory the idea of reaching definite 
results by theoretical calculations. But in spite of this difficulty hybridi- 
zation offei'S a means of creating new forms which cannot be obtained 



Pod Concave 
Near Center, 

Near Toil 



Convex Kull, Constricted 
Near Center, between Beans 
Near Stem 



Xooaely Filled, p j 

Crowded with ISeans *^uu- 

1 SDture 
(Bacls)F.ull 



Broad 
Kidney 




Ends 
Bounded 
Broad 

OEnds 
Truncate 

jDoub^e Barrelled 

Unsymmetrlcal 



Short, Curved from 
Center of Pod 



(Point 
Beans, on /K Short^ Straight 
/„ . .. ,,. ". „., from Back 
' Center Line, Alternate Sides „, n a 
01 Pod 

Outline Drawing. Checli outlines which best illustrate the general form and character of pod and 
also those showing the characteristics of the beans and their arrangement in the pod- 



ACCESSION No. 


BEAN 


VARIETY 




SOURCE 






Location ol trial 






Section 


row to row, inc. Observer 


Year 


SOIL 

Gravelly, sandy. 


loamy, 


clayey 


mucky wet dry 


PREVIOUS CROP 
early late well poorly 


FEETILIZEE USED? 
fitted 


AREA 

feet of row 


plants to ya 


rd feet between 


rows samples in trial 






CULTURE 

from seed in place 


Culti 


vation 


frequent da,.'P thorough Plants 


sprayed 


times with? 




WEATHER CONDITIONS 
hot cold • dry 


wet 


INSECT INJURY DISEASE 
much little none ranch little 


DEVELOPMENT OF 
none good fair 


PLANTS 
poor 







zr:^ 


— 



















^_ 



















Fig. 174. — Reverse of description blank shown in Fig. 173. 

otherwise. Thus it is an indispensable supplement to the methods of 
selection and strain-testing. 

Variety Tests. — The principal purposes of variety testing are first, 
establishment of varietal types of field crops and vegetable plants, and 
second, determination of the best varieties for a given locality. In the 
establishment of varietal types many difficulties are encountered of 
which we shall consider four. 



428 GENETICS IN RELATION TO AGRICULTURE 

(a) Confusion in Nomenclature. — Horticultural and agricultural plant 
nomenclature is in a state of great confusion, a condition which makes it 
necessary at the outset of scientific variety testing to make taxonomic 
studies of the known types and to adopt a system of nomenclature which 
shall be followed consistently throughout the investigations. 

(6) Inaccurate Descriptions. — The type descriptions of many cultivated 
varieties are either too meager or faulty to be serviceable in critical work or 
they are wanting altogether. This often makes it necessary to secure 
seed bearing a given varietal name from various growers to make com- 
parative trials and then to choose the particular lot which may be con- 
sidered most nearly typical of the variety. Much of this expenditure of 
time and money could be saved by the general adoption of definite, 
mechanical methods of describing varieties. The Bureau of Plant 
Industry of the U. S. Department of Agriculture has advocated for j^ears 
the wider use of variety description blanks such as the one for beans 
reproduced in Figs. 173 and 174. 

(c) Impurity of Commercial Seed. — This is one of the most serious 
diflEiculties in the way of accurate work in variety testing. Corbett 
states that practically every sample of seed of any of the turnip-rooted 
beets will be found to contain 50 per cent, or more of roots resembling 
more or less closely the form and other characteristics of the variety, 
but the rerhaining population will be made up of an admixture of all 
possible variations of the turnip-rooted beets. The condition in potatoes 
is also serious according to Corbett and before Stuart undertook by 
progeny-row and hill-selection methods to establish pure strains of com- 
mercial varieties any pretense at variety testing consisted merely in 
comparing one mixed lot with another mixed lot. 

(d) Complexity of Variation. — The fact that fluctuating variations 
may be both heritable and non-heritable makes it necessary to use pedi- 
gree culture methods in any effort to compare the genetic constitution 
of varieties. 

In determining the best varieties for a given location the chief de- 
siderata are yield, quality or chemical composition and uniformity. The 
chief requisites for success are (1) an adequate system of records; (2) 
proper interpretation of the results. To facilitate the keeping of accurate 
notes about each variety the use of loose-leaf printed forms has been found 
most practicable. Three of the forms used at the Maine Experiment 
Station are reproduced here. Fig. 175 shows a facsimile of the plot 
record blank which may be used for any crop and provides for size of 
plot, fertihzation, seed used and general notes. Fig. 176 shows a fac- 
simile of the blank used for recording data on oat varieties. Fig. 177 
shows the plot index by which it is possible to trace back the pedigree of 
any plot culture as plot numbers are never duplicated This form is 



PLANT-BREEDING METHODS 



429 



used in all the plant-breeding work at that station. These forms il- 
lustrate the sorts of records which it is necessary to keep; the sheet for 
data on varieties would have to be designed especially for each crop plant. 



§ 



Planted Harvested 


Plot No. 




Last Year In 


Plant 




Length Ft. Breath 


Ft. Area 


Aerea 


Manure 


Fertilizer 


Seed Used 


Amount of Seed Used 


Disposal of Crop 


Why Planted? 


Observer 


rnTF" f'S'B-ivi SOMEIHISG? A Blank Eage Convejt 
*■ V and ojdj Suggests the Obsen-er was Btind 


Ko IsformatioD v 





Fig. 175. — Facsimile of plot record sheet used in oat variety tests at the Maine Experiment 
Station. {After Surface and Barber.) 



Seed 






Year 


Plot 
Row 


No. 
No. 




stand 




Straw 




Heads 




Yield 




Full 




Height 




Symmetrical 




Total Weight 




*of Full 




Wealt 




Spreading 




Grain » 




Even 




Medium Str. 




Close 




Straw .. 




Uneven 




Stiff 




Side 




Bu.(32 Lb.)per A 




Maturity 




Leaves 




Branches Stiff 




Bu.(ActJperA 




Early 




Broad 




" Drooping 




Lbs. per Bu. 




Medium 




Medium 




• • Short 




Seeds in Gm. 




Late 




Narrow 




Long 




Grain 




Matured Well 




Color Dark 




Large 




White 




1. Fairly 




" Medium 




Small 




Yellow 




■ • Poorly 




" Light 




Long 




Brown 




Stcx>ling 




Disease 




Short 




Black 




Heavy 




Smut: Much 




^Spikelets Filled 




Mixed 




Medium 




Little 




i I. Barren 




Good 




Light 




Sust: Much 




Pins Enclosed 




Fair 




General Score 




•• Little 




'■ Free 




Poor 





















Fig. 176. — Facsimile of oat breeding record used in oat variety tests at the Maine Experi- 
ment Station. (After Surface and Barber.) 

The experimental errors involved in variety testing may be considered 
under two heads: (1) accidental errors such as incorrect weighing, 



430 



GENETICS IN RELATION TO AGRICULTURE 



faulty computation, unobserved variations in field treatment, sampling, 
etc.; (2) residual errors such as variations caused by soil heterogeneity due 
to natural conditions or to non-uniform treatment, uneven distribution of 
soil moisture, etc. The practical questions involved in reducing ac- 
cidental errors and the experimental determination of the probable 
error have received considerable attention particularly from English and 
American agronomists (see papers by Carleton, Farrell, Hall, Hall 
and Russell, Lyon, Mercer and Hall, Montgomery, Olmstead, Pritchard, 
Stockberger, Surface and Barber and Wood and Stratton). The need of 
some suitable mathematical criterion of soil heterogeneity has been 
pointed out by Harris. The criterion proposed is the coefficient of cor- 
relation between neighboring plots of the field. With the method of 



K 


Plot 
Number 


Year 

Planted 


Plant 


Seed Used 


Disposal of Seed Produced 














1 
























i 












i 






1 — — - — -^ 


— ' — "^Z^^^^ 


1 —- 


■c 












H 












1 












1 






















^ 

























Fig. 177. 



-Facsimile of plot index sheet used in all plant breeding work at the Maine 
Experiment Station. {After Surface and Barber.) 



treatment developed by Harris it has been shown that correlations 
between the yields of adjacent plots ranging from r = 0.1 15 +0.044 to 
0.603+0.029 can be deduced from the data of fields which have passed 
the trained eyes of agricultural experimenters as satisfactorily uniform. 
In three out of four cases tested the coefficient was more than 8 
times as large as the probable error indicating a relatively large degree of ' 
soil heterogeneity. Harris' method in condensed form is as follows: 



Add together the yields of a chosen number of contiguous p plots to form a 
number m of combination Cp plots. The sum of the squares of p is subtracted 
from the sum of the squares of Cp and the result divided by m[n{n — 1)], where 
n is the number of ultimate plots in each of the m combination plots. The 
quotient is reduced by subtracting the square of the mean yields of the ultimate 
plots p, and the remainder divided by the square of the standard deviation 
of yields of ultimate plots, a-p"^. The quotient is the correlation between the 



I'LA N T-BREEDING METHODS 



431 



yields of the ultimate units, p, of the same combinatiou plot, C „, the measure 
of heterogeneity required. If S indicates summation the formula is 

[[S{C^'')-S{v^)]/m[n{n - 1)]} —p^ 



where n is constant throughout the m combination plots. 

Surface and Pearl have devised a method of correcting for soil 
heterogeneity which when tested by Harris' method was found to give 




Fig. 178. — Head-to-row nursery (wheat) in which 25 grains from a single head are planted 
in a row 20 inches long. {After Montgomery.) 

in all cases a very marked reduction in the amount of heterogeneity when 
the corrected figures were used, and when tested experimentally, it 
seems that this method leads to results which more nearly represent the 
truth than do the uncorrected yields. However, this method in its 
present form is adapted for use only when the plots are arranged in a 
particular way which is not always practicable with certain crops or 
on certain areas. It is probable therefore that the older method of 
check plots or rows together with replicate planting will continue to be 
used. Following are some of the conclusions reached by Montgomery 
regarding the reduction of experimental error: 

(a) Systematic repetition constantly reduces error as the number of repeti- 
tions increases, but with 16-foot row plots 10 to 20 repetitions must be made, 
depending on the degree of accuracy required. 



432 



GENETICS IN RELATION TO AGRICULTURE 



(6) It is probable that the greater the number of strains to be compared the 
more repetitions will be necessary, because of the greater area they will cover. 

(c) Small blocks, 5.5 feet square, give results similar to those of the row plots, 
except that the reduction of experimental error is somewhat greater as a result 
of repetition. Blocks repeated 8 or 10 times give results apparently about as 
accurate as rows repeated 15 or 20 times. 

(d) The rate of planting within certain wide limits, has little influence on 
yield. 

(e) There is some competition between adjacent rows, especially when 
varieties very different in habit of growth are planted side by side. The use of 
blocks does away with this source of error. 



t < ' t < M I t 



" L" ' j ' ' » • « ' I » n « « t I ? i » 



♦ . I f « ' 



' ' ' ' 1 1 ■ . i:.M , i 







Fig. 179. — Row-plot nursery (wheat) in which the luws ;ut; IG feet in length with a 
4-foot alley adjacent, thus making the beds 20 feet in width. {After Montgomery.) 



if) Block plots and row plots at the usual rates of seeding will probably 
correlate more closely with results in field plots than in plots where the plants are 
spaced as in centgeners. 

(g) Where error is corrected by the system of repetition plots, check plots 
would be used for the purpose of determining the experimental error. When 
the variation in checks equals the variation in strains, no possible selection can 
be made. 

Strain Tests. — The chief purposes of strain tests are (1) separation of 
types within commercial varieties with a view to standardization of 
varieties and (2) selection of the most profitable strains within a variety. 
The mixed "condition of our varieties of vegetables calls for continual 
attention on the part of the seedsmen and experiment stations in an 
effort to bring existing varieties up to some definite standard. One 
difficulty is found in the widespread use of synonyms, according to Work, 



PLANT-BREEDING METHODS 433 

who advocates the organization of a recognized board of review which 
will decide after trial and comparison, whether a submitted sample is 
worthy of standing as a new variety or simply as a strain or stock. 
Myers has conducted extensive strain tests of tomatoes and has 
reached the conclusion that the best way to insure success in procuring high 
yielding strains of vegetables is to secure seed a year in advance of the 
time it will be needed and submit it to a preliminary test. The difficul- 
ties encountered in selecting the most profitable strains within a variety 
involve the same sources of experimental error as are met in variety 
testing. The necessity of distinguishing between heritable and non- 
heritable variations calls for individual plant selection and pedigree 




Fig. ISO.— Increase plots of one-thirtieth acre each. Selected strains of wheat from 
the nursery are tested in these plots for 3 years. {After Montgomery.) 



culture methods. Plant-to-row tests and subsequent plot tests of the 
progeny of individual wheat plants are shown in Figs. 178-181. In all 
such work the use of loose leaf record blanks is advantageous. Two 
forms of blanks used in testing pure line selections of oats are shown in 
Figs. 182 and 183. Plant-to-row strain tests are still generally used 
with cotton, corn and other cross-fertilized plants, but Hartley has 
pointed out the importance of reducing experimental error to a minimum 
in testing corn. 

Factors That Affect Experimental Results.— In discussing the 
standardization of field experimental methods Piper prepared the fol- 
lowing hst of factors affecting experimental field work with plants, 
advising that they be published in connection with any series of field 
experiments where relative yield is the object sought. 

28 



434 GENETICS IN RELATION TO AGRICULTURE 

Climatic: 

1. Character of season as to rainfall, temperature, etc. (These data are usually 
available in the Weather Bureau records.) 

Edaphic: 

2. Character of soil. 

3. Preparation of soil. 

4. Fertilizers. 

5. Cultivations. 

6. Irrigations. 
Ex-perivieiital: 

7. Size and shape of plots. 

8. Error due to marginal effect. 













»Q 


^^^R^ r^.. 


^-^fipjl 


^^m 






I^KitaH 


H|g^^;.^ 




^^m 






^^M^PR 






Hh^^^HI 






E^WfSo. 'iWBIBSBtBtlmimk 




n 


I 


^^«q 


III 


H 



Fig. 181. — Increase plots harvested and ready to thresh. The plots in this field averaged 
60 bu. to the acre. {After Montgomery.) 

9. Method of obtaining yields. 

10. Percentage of moisture at time of weighing. 
Biological: 

11. Variety of plant, including purity and trueness to type. 

12. Source of seed. 

13. Viability of seed. 

14. Preceding crop or crops. 

15. Date of seeding or planting. 

16. Rate of seeding or planting. 

17. Method of seeding or planting. 

18. Date of appearance above ground. 

19. Percentage of stand. 

20. Uniformity of stand. 

21. Uniformity of growth. 

22. Percentage of weeds. 

23. Date of blooming or heading. 

24. Date of maturity. 



PLANT-BREEDING METHODS 



435 



25. Stage and evenness of maturity. 

26. Date of harvesting. 

27. Damage by disease, animals or weather. 





Date 


Harvested Selection No. 




-a 


Planted 


By 


Line No. 


Crossed 
Selfed 




Selected from I>lot 


Row No. Plant No. 




Crossed as Mother Parent with Plant 


Row Plot Variety 




fe 


Planted in Breeding Plot No. 


Row Plant 






Oats. Variety: 




Photo Neg.No. 




il.ifeht 


Nu. of 
Culms 


Weight 


LenL'th 

of 
Head 


Type 


Color 
Grain 












4-> 


Plant 


Grain 


Straw 


Hca<l 


Groin 


R) 






























Ph 






























C 


Dent 
Corn,s),'e°e't. Variety: 




Uticht 
Plnnt 


No. of 
Ears 


Weight 


Lenijth 
Head 


Diameter 


No. of 
Rows 


Kernels 
per Row 


GermJn- 
ation^ 








r5 


Ears 


Cobs 


Corn 


Mid-Ear 


Mid-Cob 


to 






































1 




















Beans, Variety: | 




lleifhl 

of 
Plant 


Runner 
Dwarf 




Weight 


Number 


Color 
Pods 


Color 
Beans 


T^pe 
Ceans 


Tjpe 
Leaf 




.o 


Plant 


Filled Pods 


Beans 


Emptj Pods Stalk 


Pods 


Beans 


bO 






























«>J 
































Hotus: 



Fig. 182. — Plant selection record suitable for oats, corn or beans. (After Surface and 

Zinn.) 



T! 


Variety Planted Plot Row No. 


o 


Blossomed Harvested Sister Rows 


^ 


Mother Plant Row No. Plot No. Contrast Rows 




Plant 

No. 


Ilc-ifht 


Number 

of 

Culms 


WeiEht 

of 

Plant 


Weictlt 
Grfin 


Weight 

of 

Straw 


Planted 


Line No. 


fl 


Plot No. 


Row No, 


Mother Selected For 


^ 


A 
















a 


B 
















Oi 


C 
















S 


D 
















No. of Plants 


E 
















General Notes 


!3 


F 


















G 
















pa 


H 
















J 
















Is 


K 


















L 
















ri 


M 
















o 


N 


















O 
















fri 


P 


















Q 


















R 
















5 


S 


















T 
















<3 


U 
















■ft, 


















ffi 

a 


Total 
















Mean 
















St. D 
















t3 


















a 



















Fig. 183. — Progeny row record blank. (After Surface and Zinn.) 



Another biological factor is percentage of soil moisture, 
suggests the following standards for variety testing. 



Piper also 



436 GENETICS IN RELATION TO AGRICULTURE 

Minimum Standards Recommended for Varietal and Similar Tests with Corn. 

Duration of trials: Five seasons. 

Size of plots in plot-tests: Five rowo each of twenty-five hills or each 5 rods long. 
Outer two rows to be discarded. 

Length of rows in row-tests: Twenty-five hills or row 5 rods long. 

Number of checks: Every fifth plot or every fifth row. 

Number of replications: Five times in rows; at least twice, preferably 3 times, in 
plots. 

In row tests only closely similar varieties should be in contiguous rows. 

Minimum Standards Recommended for Varietal and Similar Tests with Small 
Grains. 

Duration of trials: Five seasons. 
Plot tests: 

Size of plots: 3^o to Ho acre. 

Number of replications: At least twice, preferably 5 times. 

Number of checks : Every third plot. 

Margins on outside plots : There should be a border of at least 3 feet to dis- 
card. Paths or division strips are preferably avoided when possible. 

Blocks: Square, so as to permit changing the direction of the plots from season 
to season. 

Shape of plots: Long and narrow. Each season the series of plots should be laid 
out at right angles to the previous plots. 

Previous crops : The record for 3 years should be given. 
Row Tests: 

Length of rows: One rod or more. 

Distance between rows: Six to 10 inches. 

Method of seeding: Drilled at optimum rate of seeding under field conditions. 

Rate of seeding: To be indicated. 

Checks: Every fifth row. 

Replications: Ten times. 

In row tests the outside row should always be discarded. 



CHAPTER XXVI 
GENERAL CONSIDERATIONS AND CONCLUSIONS 

In these chapters on plant breeding the primary purpose has been to 
present the methods by which breeders may make practical application 
of genetic principles. The introductory historical treatment was in- 
tentionally pragmatic in trend. It is only just however that students 
should recognize the debt which modern agriculture owes to those pioneers 
in biological science who laid the foundation for the science of genetics 
through their experimental investigations of plant hybrids. Reference 
has been made to a number of these men in earlier chapters; we may now 
briefly consider the general bearing of their work on the development of 
plant-breeding methods. 

The Relation of Science to Plant Breeding. — The influence of scien- 
tific discovery on the early history of plant breeding is not marked. The 
pioneer plant breeders, Van Mons, Thaer, Knight, Cooper, Le Couteur, 
Shirreff and Hallet, undertook the production of new and improved 
varieties, while the Linnaean theory of the catastrophic origin of all living 
things was still accepted by most scientists. Even Hovey, Sutton, 
Bull and Vilmorin completed most of their work before the publication 
of Darwin's " Origin of Species. " Thus the beginnings of plant breeding 
were made by florists, horticulturists and agronomists, who observed the 
defects in commonly grown varieties and sought to improve them or to 
find better ones. Each attacked the problem in the light of his own 
knowledge or theories, the later ones sometimes profiting by the experi- 
ence of their predecessors. 

However, while the early plant breeders were working along empirical 
lines, the first efforts to obtain scientific knowledge of plant hybrids were 
being made. The conception of sexuality in flowering plants began to be 
formulated during the last quarter of the 17th century. It was in 1676 
that Nehemiah Grew first expressed the idea that the anthers are sexual 
organs (pubhshed in 1682). According to Focke, the knowledge of sexu- 
ality in higher plants was really established by Rudolph Jacob Cammerer 
(Camerarius), whose first experiments were made at Tiibingen in 1691. 
Three years later he published his ''Epistola de sexu plantarum." Dur- 
ing the first half of the 18th century the famous Swedish botanist, Carl 
von Linn^ (Linnaeus), also experimented with hybridization in plants, 
and his cross between two species of salsify {Tragopogon pratensis and 

437 



438 GENETICS IN RELATION TO AGRICULTURE 

T. porrifoUus) which bloomed in 1759, was, according to Focke, the 
first plant hybrid to be produced for scientific purposes. But Linnaeus' 
ideas as to the possibility of hybrids between even widely different an- 
cestors were somewhat fantastic. 

Meanwhile, another investigator had been laboring on many fruitless 
experiments. Joseph Gottlieb Koelreuter laid the foundation for the 
modern study of hybridization in plants. It was not until 1760 that he 
obtained seeds from a cross between two species of tobacco {Nicotiana 
rustica 9 X N. paniculata cf ). The hybrid plants bloomed in 1761 and 
the same year appeared his paper on sexuality in plants. He called 
attention to the role of insects in cross-pollination and estimated the 
number of pollen grains. According to Focke, but little interest was 
taken in his work by others as he was decades ahead of his time. He 
accumulated data of the greatest significance on the characters of inter- 
specific hybrids. Besides Nicotiana, he worked with several species of 
Dianthus, Aquilegia, Matthiola, Melandrium, Linum, Malva, Lavatera, 
Lobelia, Datura, Lycium, Verbascum and Digitalis. 

Toward the close of the 18th and during the first half of the 19th 
century this work of Koelreuter was extended in some directions by 
other scientists, notably by Duchesne, Sprengel, Herbert and Gaertner. 
Duchesne introduced the idea of races into botany and thus helped in 
breaking down the Linnaean dogma of the constancy of species. Sprengel 
studied the relations between flowers and insects in great detail. Herbert 
interested himself in a long series of experiments with species of Erica, 
Gladiolus, Hippeastrum and Rhododendron, securing many interesting 
results. He also engaged in a discussion with Knight on fertility in 
interspecific hybrids. Focke considers this debate in the nature of an 
introduction to the later more comprehensive controversy between 
Cuvier and Geoffrey St. Hilaire. Gaertner's experimental work extended 
over several decades and in number of experiments probably he is sur- 
passed by no other hybridizer, but unfortunately his records and dis- 
cussions of results are clumsily reported and, according to Focke, their 
worth is frequently overestimated. His investigation of fertilization 
was of greater value. 

During the last half of the 19th century the scientific knowledge 
of plant hybrids was extended by a number of other investigators whose 
contributions have been thoroughly reviewed by Focke, Especially 
significant for agriculture was the work of Godron, Naudin, Nageh, 
Darwin and Mendel. Godron, by hybridization of wheat and spelt, 
demonstrated the hybrid origin of Aegilops triticoides and so destroyed 
the old tradition that spelt had been transformed into wheat. Naudin 
opposed the conception, still maintained by Cuvier, that species are hard 
and fast entities and, based on his experiments in hybridizing different 



GENERAL CONSIDERATIONS AND CONCLUSIONS 439 

species of the Nightshade Family, he actually discovered, according to 
Blaringhem, the essential principle of Mendclism, which he expressed as 
follows: "La disjonction des deux essences specijiques a lieu dans le pollen 
et dans les ovules de Vhybride." To Nageli we are indebted for the first 
scientific treatise on hybridization that was wholly impartial and coherent. 
His works served as a common source for most of the later discussions 
of plant hybrids. Meanwhile Darwin had organized a great mass of 
information bearing on the general subject of adaptation and had dis- 
covered one general principle of evolution, viz., the principle of natural 
selection. The publication of Darwin's discovery was at once stimulating 
and deterring. It was stimulating to argumentative controversy as well 
as to certain students of heredity, notably Hoffman, who conducted nu- 
merous experiments from 1855 to 1880, and Galton, whose work during the 
80's laid the foundation for the biometrical method of treating the data 
of genetics. Darwin's theory was deterring in its effect on a further wid- 
ening of the biological horizon, at least so far as the theory of evolution 
was concerned. Biologists were so well satisfied with his conception 
that all minute, fluctuating variations are inherited and so may be pre- 
served by natural selection, that but little real progress was made in the 
study of evolution until the rediscovery of Mendel's discovery in 1900. 
The work of Mendel, although unappreciated by Nageli and other con- 
temporaries who knew of it, was destined to revolutionize the study of 
heredity. By his critical experiments and keen interpretations of the 
results of those experiments, Mendel laid the cornerstone of the foun- 
dation for the future science of genetics. During the 19th century, 
plant and animal breeding was in progress in various countries and the 
reported observations of many experimentalists on a multitude of living 
forms presented an array of diverse and apparently contradictory phe- 
nomena, the classification of which under a few natural laws was hardly 
considered. 

With the announcement of the discoveries of Mendel, de Vries and 
Johannsen during the first three years of the present century, there was 
a great .awakening of interest among biologists in the problems of varia- 
tion, heredity and evolution. In the enthusiasm of the hour it was 
thought by some that the application of these laws of heredity and muta- 
tion in practical breeding would be comparatively a simple matter. 
Many plant breeders went zealously to work only to obtain further con- 
flicting and disconcerting results. At the same time research students 
the world over began new investigations on variation and heredity. 
The occurrence of mutations and the existence of pure lines in species 
that reproduce by self-fertilization only have been verified; but to ex- 
plain the heredity of most plants requires certain modifications or ex- 
tensions of the three original Mendehan "laws." The investigations 



440 GENETICS IN RELATION TO AGRICULTURE 

which have determined the nature of these modifications and extensions 
are very recent and they involve considerable technical knowledge of 
biology, so that at present the progress of research in genetics is far in 
advance of the practical application of the principles now known. How- 
ever, some of the fundamental principles of genetics have become avail- 
able to the practical plant breeder with the result that much unnecessary 
waste of time and labor has been prevented and that results have become 
more certain in some cases. 

The Future Relation of Genetics and Plant Breeding. — The extent 
to which factorial analysis has been carried in the cultivated snapdragon 
and the garden pea is sufficient to indicate what may sometime be accom- 
plished with plants of greater economic value. East has shown the im- 
portance of knowing the chromosome number of a species before planning 
extensive breeding operations, but in many of our important crop plants 
the chromosome number has not yet been determined. The development 
of technical plant breeding along definite genetic lines will follow the work 
of purely scientific discovery, but it must develop more slowly because of the 
greater length of time required and the expense entailed in the application 
of genetic principles to crop plant improvement. However, it is not too 
much to expect that eventually our more important crop plants at least 
will have been subjected to such thorough germinal analysis, that the es- 
tablishment of desired strains will become largely a matter of reference to 
breeding records and the repetition of certain crosses and selections. 
In other words, it is probable that the improvement of our important 
seminally reproduced crops will have become so well systematized as to 
make it possible to predict the outcome of crosses between recognized 
types, as well as the behavior of new mutants. This factorial analysis 
must apply to quantitative as well as qualitative characters. Even the 
discovery that certain characters of economic importance are conditioned 
by too many factors to make the production of new desired combinations 
probable except in very extensive cultures, should prove of direct value 
to agriculture. If the new form is sufficiently desirable the combined 
resources of several experiment stations or other agencies might be con- 
centrated upon its production. There must be closer coordination of 
breeding projects for the purpose of avoiding needless duplication and 
insuring more rapid progress. 

The Importance of Planning Breeding Operations in the Light of 
Scientific Knowledge.^ — It is maintained by some that two separate and 
distinct branches of breeding should be recognized, viz., conservative 
breeding and constructive breeding. The first is supposed to preserve 
and utilize the desirable characters already in existence; while the second 
attempts actually to improve the characters of plants and animals. But, 
as Cook has shown, there is little to support the popular idea that the 



GENERAL CONSIDERATIONS AND CONCLUSIONS 441 

operations of breeding result in "new creations" in the sense of bringing 
new characters into existence outside of those already attained in the 
course of evolution. Moreover, it would be very difficult to maintain a 
distinction between conservation and construction in modern breeding. 
A scientifically planned system of breeding improved crop plants must 
be founded upon suitable methods of testing the species, varieties and 
biotypes available in order to discover the most promising material 
for the constructive work of the hybridist. 

Uneconomical methods must be eliminated. There are sources of 
enormous waste in present day plant breeding work. An example is 
the useless attempt at improving pure lines through continual selection; 
another is the assumption that a single pure line selection represents all 
there is in a variety, a serious mistake since most commercial varieties 
of self-fertilized plants consist of a mixture of pure lines. The frequency 
of mutations in all economic plants has a direct bearing upon these ques- 
tions of breeding practice; hence this is a subject which deserves more 
thorough investigation. The evidence in some species is rather definite, 
however. In the potato, for example, it is probable that bud mutations 
are very, very rare. Yet the idea still prevails that disease resistant 
strains of commercial varieties of potatoes can be obtained by hill selection 
methods. In a variety susceptible to a given disease such strains would 
have to originate as bud mutations and, while it is possible of course that 
such a mutation in a given variety might occur, still in the light of what 
is now known about the rarity of bud mutations in the potato it is prob- 
able that in order to locate such a strain it would be necessary to test 
millions of plants under conditions favorable for the disease. Severe 
epidemics occasionally furnish opportunity for such selection on a grand 
scale. But the scientific plan of procedure is to undertake variety test- 
ing on a large scale preparatory to hybridization of the most promising 
forms. 

The matter of adjustment between varieties and local environmental 
conditions is of considerable importance. This is more widely recognized 
in cotton perhaps than in any other crop. Cotton growers are generally 
advised to secure locally grown seed, provided it has been properly selected 
and handled. Unless recourse is had to the production of Fi hybrid 
seed, this consideration of adaptation to local environment is destined to 
become increasingly important as greater improvement is sought through 
more intensive selection. For this reason seed production will probably 
become more localized even though the business of handling and retailing 
remains in the hands of comparatively few commercial establishments. 
The recent rapid development and localization of truck crops in the 
United States as reported by Blair is a case in point. Specialization 
of this sort is bound to increase along with increase in population and 



442 GENETICS IN RELATION TO AGRICULTURE 

the utilization of superior locally adapted varieties or strains will become 
correspondingly important. 

More comprehensive study of all the factors involved in a plant breed- 
ing problem will be demanded of future plant breeders. Not only must 
the inheritance of the economic characters of each crop plant be deter- 
mined, but also the important desiderata of correlation between these 
characters as derived from biometrical studies such as Harris' on the 
physiology of seed production will need to be considered. Variety and 
strain tests must become more comprehensive and at the same time more 
specific as regards standards of selection. The data on disease resistance 
especially should receive more particular attention. Finally the mathe- 
matical adequacy of experimental data derived from breeding investi- 
gations is a matter requiring the most serious consideration. 

The successful plant breeder will not only approach his problem from 
a scientific point of view and with a knowledge of genetic principles; he 
will be conversant with the developing requirements of 20th cen- 
tury agriculture. If he would do his share in the creation of new and 
more efficient types of crop plants, he must utilize the facts brought to 
light by botanical, physiological, agronomic and horticultural investi- 
gations. Of course there will always be the chance of accidental dis- 
covery and the empiricist who operates on a large enough scale will 
occasionally obtain valuable results. But the scientific plant breeder 
of the future should combine the qualities of investigator and practical 
agriculturist. The field is almost unlimited. During the 50 years 
preceding the war plant breeding had increased the yields of crops in 
Germany about 25 per cent. There were forty breeders of rye, seven- 
teen breeders of potatoes, sixty of oats, and so on with the important 
crop plants. When we consider the extent and diversity of agriculture 
in America and the low average production per acre in most of our im- 
portant crops, it is evident that the plant breeder has abundant oppor- 
tunity. Yet it must always be remembered that the full possibilities 
of applying genetics to breeding problems must await the gradual devel- 
opment of scientific research. 



PAET III— ANIMAL BREEDING 

CHAPTER XXVII 
THE GENERAL ASPECTS OF ANIMAL BREEDING 

From a scientific standpoint it would be practically useless in this 
treatment of genetics in relation to animal breeding to develop exten- 
sively the historical features of the subject; because they cannot be re- 
lated effectively and satisfactorily to a growing knowledge and application 
of the principles of variation and heredity, and because of the peculiar 
nature of many of the problems of animal breeding. Accordingly this 
chapter will be devoted for the most part to a discussion of the importance 
and possibilities of the breeding industry, and of the opportunity for 
service which genetics has therein. 

The History of Animal Breeding. — The domestication of animals 
occurred very early in the history of man; so early that accurate historical 
documents do not carry us back within sight of the time when man first 
began to take wild animals under his care. The history of most of our 
domesticated animals, in fact, is very incomplete, and in many cases we 
can only conjecture as to the wild species which were probably subjected 
to domestication, or from the hybridization of which our tame breeds 
have had their origin. This difficulty of determining precisely what 
wild species have been utihzed by prehistoric man, or in finding among 
wild species any which are obviously closely related to those under do- 
mestication, is in itself proof conclusive that improvement in herds of live- 
stock, kept at first perhaps in a state of semidomestication only, must 
have been coincident with the beginnings of domestication. Through 
long centuries of slow progress the level of excellence in early tribal herds 
had gradually been raised, partly by the action of factors unknown to 
and undirected by primitive herdsmen, partly under his conscious 
direction. As a result man has established numerous races more defi- 
nitely suited by far to his particular purposes than were their wild 
ancestors which roamed the plains or inhabited the forests. Conse- 
quently even at the dawn of history, domesticated animals had already 
been developed to a high state of excellence, when measured by their 
adaptability to particular local conditions of life and their suitability 
for the purposes for which they had been bred. 

443 



444 GENETICS IN RELATION TO AGRICULTURE 

During historical times constant improvement has been made in the 
material thus provided by the early herds and flocks, a century sometimes 
sufficing for the establishment of a new breed of very superior excellence. 
Seemingly these have been remarkable achievements, but we must never 
forget when we consider them that they have been associated almost 
invariably, particularly those which have been most striking, with 
changes in the conditions of life of man himself and the purposes for 
which he has employed his animals. The Arab, nomadic inhabitant 
of the desert, needed for his purpose a horse of speed and stamina, a single 
favorite steed sufficing for each individual. Constant association be- 
tween master and mount developed in the Arab that high pride in the 
excellence of his steed, a most commendable characteristic of the desert 
dwellers of Arabia. We find, therefore, that the horses of these peoples 
are superior in intelligence, stamina, and beauty of form to those of almost 
any other land; we find them with pedigrees carefully kept and tracing 
back to the seventeenth century before the Christian era. According 
to reports of Upton, who lived among the Anezah Bedouins, famed even 
among the Arabs for the superior excellence of their horses, no animal 
was recognized as pure bred which did not trace back to the five mares Al 
Khamseh of Sheik Salaman; and the descendants of these five mares 
are divided and sub-divided into an intricate system of families and sub- 
families. But the modern French farmer with a settled mode of life 
needed horses for different purposes, primarily for drawing implements 
of tillage. Accordingly he took horses of the old draft type, large, 
rawboned, and heavy of weight, but not high in quality or energetic 
in disposition and crossed them with Arabians, Barbs, and Danish horses; 
and it was not long before all the neighboring regions of France and Ger- 
many were demanding horses from La Perche. No long period of im- 
provement was necessary for the establishment of the Percheron breed; 
the excellent qualities which it possessed were contained within the old 
breeds which existed at that time; the improvement was merely a re- 
arrangement and blending of existing qualities in a form to meet the par- 
ticular demand of modern rural conditions. Such a breed would have been 
of doubtful value to the barbaric races which swarmed over Europe 
over a thousand years ago, but for the life that those races lead today, it 
and other breeds possessing similar utilitarian advantages are performing 
a tremendous agricultural service. It would be possible to recount 
similar cases of breed improvement in all kinds of domestic animals. 
Fundamentally practically all these instances agree in this respect that 
when breeds have been established within a relatively short period of 
time, potentialities have been made use of which already existed in the 
foundation stock. Translated into the more precise terms of genetics 
this statement would imply that the hereditary material of modern 



THE GENERAL ASPECTS OF ANIMAL BREEDING 



445 



breeds of livestock is made up of elements drawn from that variety of 
material which constituted the early foundation stock; the hereditary 
material for the most part represents a recombination of factors already 
existing rather than discovery and utilization of new factors. 

The Animal-breeding Industry. — Any just conception of the extent 
and magnitude of the animal-breeding industry can only be gained by a 
consideration of the statistics of animal industry. In crude form these 
are given for the United States in Table LVII. The total number of the 
major kinds of livestock on date January 1, 1916, exceeded 200,000,000. 

Table LVII. — Estimated Number and Value of Livestock on the Farms of the 
United States on January 1, 1916 



Kind of stock 


Number 


Average value Total value 


Horses 


21,166,000 
4,565,000 
21,988,000 
39,453,000 
49,162,000 
68,047,000 


$101.60 

113.87 

53.90 

34.49 

5.17 

8.40 


$2,150,468,000 
519,824,000 


Mules 


Dairy cattle 


1,185,119,000 


Beef cattle 


1,321,135,000 


Sheep 


254,348,000 


Swine 


571,890,000 


Totals 


204,381,000 


6,002,784,000 



Their total value which was perhaps slightly augmented by war conditions 
obtaining at that time was in excess of $6,000,000,000. Animal 
industry on an average contributes over 35 per cent, of the total 
income of the agricultural industries. These totals when examined 
closely show how great is the need and opportunity for improvement. 
For in this connection it is the average value which is of most importance, 
and average values as given here do not reflect much credit on the quality 
of livestock in the United States as a whole. 

Particularly is this true when the average value or production is com- 
pared with the high-water marks which have been reached within the 
past few years. Thus to consider a matter upon which we can get fairly 
specific statistical data of value as a basis of comparison, the average 
production of the dairy cow in this country is about 3500 pounds of milk, 
yielding about 150 pounds of butter. The figures are not very accurate, 
but they are sufficiently so for purposes of comparison. We may compare 
this figure with the records which have been made by pure-bred cows of 
dairy breeds. Thus the Holstein-Friesian record is above 30,000 pounds 
of milk, and is very nearly 1500 pounds of butter. The Jersey record 
stands at about 1200 pounds of butter, and other breeds are not far be- 
hind. The Holstein-Friesian cow Tilly Alcartra produced almost as 
much milk and butter in 1 month as does the average cow in 1 year. 



446 



GENETICS IN RELATION TO AGRICULTURE 



From an economic standpoint, the importance of high production must be 
emphasized, because it is closely associated with economy of production. 
Fig. 184 based upon 443 yearly farm records of dairy cows shows clearly 
how closely net income is dependent upon high yield. 

The comparison of other pure-bred livestock with the general average 
is not so direct, but is sufficiently striking. The great trotting sire, Peter 
the Great, with 230 performing offspring to his credit, forty of which had 
records of 2:10 or better, at 21 years of age sold for $50,000. He 
was considered so valuable that his service price was placed at $400. 




4 5 6 7 8 9 10 

Thousands of Lbs. of Milk per Year 

Fig. 184. — Relation of yield of cow to feed cost of milk. {From 1915 Yearbook, U. S. 

Dept. of Agr.) 



Thirty-two pure-bred Percheron horses sold at auction for an average of 
$705 per head. A half interest in the Percheron stallion Carnot was sold 
in recent years for $20,000. Overton Harris sold at public auction 61 
head of Hereford cattle at an average price of $1246; and at about 
the same time the American Hereford Breeders' Association sold 45 
head at an average price of $1005. Within the last few years two yearling 
Holstein-Friesian bull calves have changed hands at public auction at 
$20,000 each. Forty-six head of pure-bred swine were sold at an average 
of $214 each. These figures, all of which are of sales which have taken 
place within recent years, are far above the average of the breeds which 
they represent, but they are by no means isolated i-ecords. The average 
auction price of pure-bred beef cattle for breeding purposes is at least 
five times as high as the average farm value of beef cattle, and about 
the same ratio probably obtains in other breeds of livestock. The figures 



THE GENERAL ASPECTS OF ANIMAL BREEDING 447 

testify eloquently to the opportunity for improvement which exists in 
the livestock breeding industry of the country. 

The Art of Breeding. — As an inevitable result of the years of careful 
management to which livestock has been subjected, there has grown up a 
considerable fund of empirical knowledge having to do not only with the 
best methods of caring for and feeding animals, but also with the best 
methods of mating them to ensure the production of the proper type of 
offspring. Many systems of breeding have been subjected to rigid 
practical tests; tests which have been duplicated and reduplicated in 
single breeds and in different breeds. Consequently, although many 
mystical ideas have often survived over long periods and although 
some still have their following among practical men, particularly if they 
happen to have been championed by breeders of outstanding success, 
nevertheless the tendency has been slowly, but surely, to separate the 
true from the false. Animal breeding practice in its best form has 
reached an exceedingly high state of development ; the old herdsmen who 
have grown up among their livestock, although their scientific training 
may be very limited, are masters of the art of breeding. Like artists in 
general they do not need to know very much about the composition of 
the materials with which they work ; what they do need to know, and in 
truth what some of them do know marvellously well, is how to utilize 
the materials to the best advantage. 

The Problems of Animal Breeding. — Here we may be permitted to 
digress a moment in order to emphasize the fact that the problem facing 
the animal breeder is different from the one facing the plant breeder. 
There are many reasons for this fact some of which it may be well to state 
here in order that no misunderstanding as to the general applicability 
of the laws of variation and heredity may arise. In the first place in 
plant breeding we are more particularly concerned with questions of 
local adaptation and matters of kindred nature. Plants are notoriously 
susceptible to differences in the environment because of their close re- 
lationship to conditions of soil and climate. It is a familiar experience 
to find that varieties of plants of proven worth in one locality fail miser- 
ably to live up to their reputation in some different region. Now to a 
certain extent this is true also of animals, but it must be patent to anyone 
that livestock is on the whole relatively independent of environmental 
influences. Man himself has migrated into new regions from the begin- 
ning of time, usually taking his herds with him. Inclemencies in the new 
surroundings have been met by construction of rude shelters or by seasonal 
emigrations. In the present time the construction of suitable shelters 
is, indeed, a universal practice, so that today domesticated animals ex- 
hibit an independence from the environment almost as great as that of 
man himself. As a consequence of the lesser need of considering envi- 



448 GENETICS IN RELATION TO AGRICULTURE 

ronmental conditions, therefore, those breeds of hvestock which have been 
improved in older locaUties have been iitiHzed in the newer agricultural 
regions such as those of the United States with little if any impairment of 
their superior excellence. We see this fact expressing itself in the importa- 
tion of large numbers of animals representing the established breeds of other 
countries; Shorthorn cattle from England, Jersey cattle from the island of 
Jersey, Percheron horses from France, and many other notable examples. 

In the second place questions of expediency intervene, and this is 
particularly true when the larger domesticated animals, horses and cattle, 
are considered. It is usually a simple thing for a plant breeder to grow 
a thousand individuals in order to try out some idea, but it is out of the 
question for a practical animal breeder to do so. Ordinarily his system 
of breeding is dictated by the rigid requirements of the highest total 
result, he cannot like the plant breeder seek for the one individual 
among thousands and then satisfied at finding it discard the rest. Too 
few generations can be obtained in a limited time, too great expense 
attaches to the raising of progeny which must finally be for the most 
part rejected, and too great difficulty arises from the universal occurrence 
of bisexuality among domesticated animals for him to attempt to follow 
the methods of the plant breeder. 

In the third place, and this perhaps is the most important item, animal 
breeding has progressed to a higher relative state of excellence than plant 
breeding. With practically all domesticated animals the herdsman has 
known individually every animal under his care, not only from the stand- 
point of individual excellence, but with respect to ancestral worth as well. 
Famous individuals have arisen from time to time the merits of which 
have attracted the attention of all herdsmen interested in the breeds to 
which the animals have belonged, and if they proved to transmit their 
good qualities in any degree, advantage has been taken of the best pos- 
sible matings to insure the perpetuation of those qualities. This process 
has gone on to a notable extent in some breeds and with remarkable 
results; in some breeds it is estimated that not more than 5 per cent., 
or one individual in twenty, of early animals is represented in the pedi- 
grees of animals living today. The inevitable result of such methods 
has been to raise the level of the breed to a very high plane, to a position 
where the only means of improvement lies in a consideration of the finer 
points of function and conformation, and in methods of maintaining 
more rigidly the high standards which have been erected. These then 
in the main are the problems which confront breeders of the best types 
of livestock; and they are problems, which we may admit frankly, 
have been handled admirably by the more proficient livestock breeders. 

The Service of Genetics. — The geneticist, whether laboratory investi- 
gator or philosophical theorist, cannot but admire the excellence of the 



THE GENERAL ASPECTS OF ANIMAL BREEDING 449 

great body of experience which has grown up from the constant apphca- 
tion of the method of trial and error in animal breeding. The art of the 
breeders' craft is not a thing to ridicule, for measured by the rigid test 
of results it abundantly justifies itself. The geneticist with all his 
knowledge of natural law and principle could not successfully compete 
with the practical breeder in the attainment of a definite standard of 
excellence, unless he added to his technical training a fund of practical 
detail. It is moreover too early in the science of genetics rigidly to 
lay down rules of procedure, particularly if those rules at any point are 
at variance with the established mode of practical procedure. It is too 
often the case, as any geneticist will be forced to admit when he reads 
accounts written not more than half a decade ago on the application of 
the principles of genetics to livestock breeding, that proper allowance 
is not often made for the future expansion of our knowledge of genetics 
itself neither with respect to the extent to which it will go nor the direc- 
tion which it may take. From time to time in the chapters that follow, 
we shall have occasion to point out how later developments of the science 
have given room for beliefs formerly scoffed at. 

What then is the service of genetics to practical breeding? Clearly 
the answer to this question lies in a consideration of the fundamental 
contrast between the science and the art of breeding. The object of both 
when applied to practical breeding is to attain and maintain a definite 
standard of excellence. The standard of excellence is the same, at least 
there is no good reason why it should not be identical in both cases. 
With respect to this goal, the art of the breeder merely outlines how it 
may be reached by the utilization of a system of rules of procedure based 
on the results of experience. The science of genetics seeks for the natural 
laws operative in the attainment of standards in general, and in discover- 
ing them of necessity includes in its findings the methods by which they 
may be attained. Obviously the methods of attaining standards are by 
no means dependent upon a knowledge of the underlying principles; 
but they may and in this case evidently have run far ahead of scientific 
knowledge. The service of genetics hes, therefore, in the clarity of 
thought which it promotes, just as knowledge of principle always rein- 
forces art. This then is the service to the skilled breeder, it tells him 
why his methods give the success they do, why some things are true 
and others false, and in case anything is rejected, which is occasionally 
done because of the stubborn tenacity of some erroneous ideas which by 
their very construction are difficult at one time both of verification and 
refutation, it endeavors to describe in terms of the operation of natural 
laws and principles the actual conditions which obtain, and which are 
responsible for the erroneous beliefs. 

29 



450 GENETICS IN RELATION TO AGRICULTURE 

The Service of Genetics in Education. — Up to this point, the discussion 
of the general aspects of animal breeding has been based upon the prac- 
tical methods of procedure followed by the most successful animal 
breeders. The rules of procedure which they follow have been handed 
down from herdsmen to herdsmen, they are the traditions of the art of 
animal breeding. They have of course always been modified in directions 
which the genius of each herdsman may dictate, for indeed much of the 
success of particular breeders has depended upon the aptitude which they 
as individuals have shown in dealing with problems of the moment. But 
these rules of breeding and the methods of employing them to best ad- 
vantage are not known to any great proportion of the animal breeders of 
any rural districts. In the United States it is estimated that the number 
of pure-bred livestock in any state does not greatly exceed 2 per cent, 
of the total number. Now not even all breeders of pure-bred livestock 
have attained to the high standard of perfection which we have employed 
as the basis of discussion in this chapter. To the great body of breeders 
even the empirical rules of practical breeding, therefore, are either un- 
known or imperfectly understood. 

In considering the service of genetics to this class of animal breeders 
to whom the best practical methods are merely a mass of confused detail, 
and to the prospective animal breeder who is just approaching the subject, 
we must take into account certain pedagogical principles. It is a sound 
major premise that any established mode of procedure in animal breeding 
must of necessity owe whatever measure of success it achieves to its 
conformance with the operation of underlying natural laws. It is also 
true that a knowledge of these underlying laws, by providing a common 
explanation for rules of procedure which at first sight appear unrelated 
and sometimes positively contradictory, tends to simplify the task of 
learning and applying the proper methods in actual breeding operations. 
To the novice, therefore, a thorough grounding in the principles of varia- 
tion and heredity provides the firm foundation to which he may later 
add the superstructure of a complete practical knowledge. Since it is 
much easier to remember related things than to hold in mind a confusing 
mass of technical detail, the novice who has a thorough grounding in 
principle may, by constantly searching for the reason in every new detail 
of procedure dictated by experience, so bind every fact to his interrelated 
body of principle that law and empirical procedure form together an effec- 
tive, coordinated working equipment. 

The Personal Equipment of the Animal Breeder. — Now that we have 
considered some of the outstanding features of animal breeding as related 
to genetics, we may well go on to a consideration of the method of attack 
in seeking to apply the principles of genetics, whether with a view to 
harmonizing existing procedure with them or in an eiffort to use them as a 



THE GENERAL ASPECTS OF ANIMAL BREEDING 451 

guide in the acquirement of that fund of practical knowledge necessary 
to successful breeding. By this time no dou])t many of the facts of 
heredity which the student has learned must have suggested ideas of 
practical utility, but at the risk of stating some truths already obvious, 
a brief consideration may be given to some cardinal features which must 
be taken into account in considering the relation of genetics to animal 
breeding. These things must be known in order to make proper use of 
the principles of genetics in practical animal breeding; they are mentioned 
here in order that it may be properly understood that this text does not 
pretend to be a complete manual of animal breeding, but merely en- 
deavors to point out the fundamental relations existing between genetics, 
as a pure science, and animal breeding, the craft or art of improving 
animals and maintaining present standards of excellence. 

Foremost among the requisites of a successful animal breeder must 
be the intimate knowledge and experience that comes from actual personal 
contact with livestock. The success or failure of animal breeding opera- 
tions often depends on little things, which, if neglected, destroy eventu- 
ally all the results of the most carefully laid plans. The Bates-Duchess 
line of Shorthorns were at one time far famed for excellence of con- 
formation, but a neglected tendency to barrenness along with close 
breeding resulted eventually in the extinction of this superior line of 
Shorthorn cattle. The method of breeding employed in perfecting this 
famous family was by no means one which from ts very nature from 
the beginning doomed the line to extinction ; on the contrary, it is one 
which gives the greatest possible degree of success provided it is applied 
intelligently and with a full appreciation of its consequences for evil 
as well as for good. The intimate knowledge which a breeder has of 
his herd should include a knowledge of every individual in it. He should 
know not only the good and bad points of the individuals but also how 
these points are related to those of their immediate ancestors. A breeder 
is on the highway to success when he is so well acquainted with the 
animals of his herd that he can tell from what immediate ancestor has 
come for instance a tendency to weakness of pastern, to a sluggish dis- 
position, or ugliness or unwillingness under strain, and similarly for the 
thousand and one things which must be taken into account consciously 
or unconsciously in all breeding operations. For minute differences as 
well as greater ones are heritable, even though as yet they have not 
been reduced to Mendehan formulation. It would be the height of 
folly for an animal breeder to call in a geneticist, however well trained, 
to map out his matings for him. The services of the geneticist can only 
be in giving the principles involved in breeding; the apphcation must be 
left to the breeder himself, who must temper his theoretical knowledge 
with an abundant fund of practical detail. 



452 GENETICS IN RELATION TO AGRICULTURE 

There is another very important prerequisite for success in animal breed- 
ing which comes in part from training, in part from the native abihty of 
the breeder himself, and that is the erection of a true and attainable ideal. 
It is not necessary here to outline fully the various factors which must 
be taken into consideration in building up an ideal, but that type when 
it has become fixed in the breeder's mind, and his breeding can hardly 
be systematic until his standard has been established, must be within 
the limits of attainability of the breed with which the herdsmen is work- 
ing, and it must also be a superior type of that breed designed to fill better 
than any other some definite economic demand. All these are matters 
with which genetics properly is not deeply concerned, but they make up, 
nevertheless, a very definite portion of the subject matter which is in- 
cluded under the term animal breeding. We see therefore that the pur- 
pose of an account of the relation of genetics to animal breeding is de- 
finitely circumscribed, it is to point out the significance and operation 
of the laws of genetics in animal breeding, not however to provide a com- 
plete compendium on this latter extensive subject. 



CHAPTER XXVIII 
VARIATION IN DOMESTIC ANIMALS 

Half a century ago when Darwin found it necessary to demonstrate 
the widespread existence of variation, he selected as his most convincing 
evidence the variability which occurs among domesticated animals and 
plants. In "The Variation of Animals and Plants under Domestication" 
he has given us a masterly, and at the same time delightful, account of the 
extreme variation which is exhibited by domesticated breeds of livestock. 
Even today although we cannot accept the explanations which Darwin 
offered to account for these variations, this treatise remains the best gen- 
eral account of variation among farm animals and household pets. But 
since Darwin's time the point of view has shifted from the question of the 
occurrence of variation, now universally accepted as an estabUshed fact, 
to the problem of the sources and causes of variation, a problem about 
which we still have much to learn. 

The Sources of Variation. — ^With regard to their relations to each 
other and their specific causes, our knowledge of variation in domestic 
varieties of animals is unfortunately considerably circumscribed. Since, 
however, it has been demonstrated that variability among all living 
beings arises from the same general sources, we may with confidence 
state that among domestic animals, as among other living forms which 
have been studied in greater detail, variations may be classified with 
respect to source under three primary heads: somatic modifications, 
germinal recombinations, and germinal alterations or mutations. More- 
over, the behavior within these groups among farm animals is strictly 
typical for the class in question. Somatic modifications arise from en- 
vironmental causes, and they are merely transient; they leave no impres- 
sion, whatever, on the germ-plasm. Variation by germinal recombina- 
tions arises from amphimixis, and in domestic animals, we have a growing 
body of evidence in support of the belief that such recombinations uni- 
versally follow strictly the Mendelian law of segregation. Definite, 
authentic cases of mutational changes in higher animals are exceedingly 
rare, but those which we have leave no doubt that they involve single 
locus alterations in the germinal material in a manner strictly analogous 
to that of mutation in the fruit-fly. Of all these kinds of variation, there 
are good isolated examples among domestic animals, but very often 
there is a deplorable lack of detail about problems which offhand appear 

453 



454 GENETICS IN RELATION TO AGRICULTURE 

to be very simple and easy of solution. This deficiency will undoubtedly 
be remedied as the results of definitely planned experiments become 
known; for the present it is necessary to make the most of the meagre 
data now at hand. 

Selection as a Cause of Variation. — It is unnecessary to reopen here 
the question of the causal connection between variation and selection, 
for the arguments which have been presented in various places for the 
belief that modifying factors, rather than quantitative changes in a given 
Mendelian factor, are responsible for changes in a given character by 
selection apply just as well to domestic animals as to any other living 
beings. It is, however, true that a large number of the characters of 
farm animals are of a type such that they may be shifted with compara- 
tive ease in a given direction by selection. The characteristic white- 
face pattern of Hereford cattle behaves as a unit in heredity, but unques- 
tionably there is sufficient evidence to show that by continued selection 
the extent of the white area may be increased or decreased in exactly 
the same fashion that Castle by selection was able to increase or decrease 
the amount of the black pigmented area in hooded rats. No experiment, 
however, has yet been carried out, or is likely to be undertaken, in 
which over 30,000 Hereford cattle have been bred and raised for the 
purpose of determining definitely what are the limits of selection for 
this character. Similarly in such breeds of cattle as the Ayrshire and 
Holstein-Friesian the proportion of pigmented area to non-pigmented 
may be shifted at the will of the breeder from a solid pigmented condition to 
one almost entirely white. With more complex characters such as speed, 
milk production, and others of utilitarian value the evidence is even more 
convincing that selection does gradually shift the mean of the race. 
Whatever may be the true interpretation, there can be no question that 
the breeder may guide the variation of nearly every character in a definite 
direction by proper methods of selection. 

Variation by Modtfiability. — The success of the art of the caretaker 
and feeder in animal breeding depends upon the existence of a high degree 
of modifiability in domestic animals. Extreme cases such as the effect of 
starvation contrasted with the effect of liberal feeding are easily recog- 
nized, but in more obscure cases it is difficult to state how much of a given 
effect is due to inheritance and how much to modifiability. Thus the 
high standard of present racing records does not depend solely upon more 
careful attention to the selection of breeding stock, for improvement in 
methods of feeding, care, and training has been associated with this 
greater discrimination in the selection of breeding stock. The same fact 
is true in even greater degree with respect to the marvellous records which 
have been made by dairy cows during recent years, for elaborate methods 
of development and feeding have been devised to stimulate production to 



VARIATION IN DOMESTIC ANIMALS 



455 



the highest possible degree. It is necessary, therefore, to exercise unusual 
judgment in comparing the records of recent years, whether of race track 
or dairy, with those which have been made a number of decades ago. 

With reference to dairy cattle it is possible to make some very interest- 
ing comparisons, first of old records as compared with those of the present 
day, and second of records of cows of uncertain breeding at the present 
time with those of cows of established dairy breeds. Thus Pearl has 
unearthed the record of a scrub cow owned by Mr. George A. Scott of 
Nashville, Tennessee, which in 1863 produced about 12,450 pounds of 
milk. The record of an Old Sussex cow for the 5 years beginning in 1805 
is given in Table LVIII. In recent years a grade Jersey cow produced 
in 1 year 16,286 pounds of 

milk, the butter fat con- Table LVIII. — Production of Milk and Butter 
tent of which, 844.8 pounds, ^^ ^ ^ow of the Old Sussex Breed, 1805- 

was equivalent to 1056 "^ ^^ 

pounds of 80 per cent, 
butter. At the time this 
record was made, it had 
been exceeded by only 
four cows within the Jersey 
breed itself. Scrub cow 
No. 131 in the government 
herd at Washington, an 
old cow between 15 and 
20 years of age, was in 
milk continuously from 
October 6, 1909 to August 

1, 1913, during which time she produced 33,066 pounds of milk. It 
appears, therefore, clearly to be established that so far as milk yield goes 
much of the improvement of late decades may have depended upon 
better methods of care and feeding; for over a century ago cows of very 
great excellence in this respect were produced occasionally, and at the 
present time cows of mongrel breeding may sometimes exhibit high 
performing ability. Undoubtedly, however, there has been an enormous 
multiplication of the best yielding families during recent years, even 
if there may not have been any actual increase in dairy potentialities. 

Obviously modifiability may act in a variety of ways. An interesting 
specific instance in dairy cattle is reported by Kildee and McCandlish. 
At the Iowa Station a comparison was made between seven cows of 
mongrel breeding, and reared under unfavorable environmental condi- 
tions, with seven calves, their offspring, of the same type of breeding 
reared under favorable conditions since birth. The seven developed 
scrubs, that is, those which had been reared under favorable conditions 



i'ear 


Weeks 
in milk 


Pounds 
of milk 


Pounds 
of butter 


First 

Second 


48.0 
45.5 
51.5 
42.5 
48.0 


10,580 
8,895 

12,367 
9,071 

11,543 


540 
450 


Third 


675 


Fourth 


466 


Fifth 


594 






Totals 


235.5 


52,456 


2,725 


Averages 


47.1 


10,491 


545 



456 



GENETICS IN RELATION TO AGRICULTURE 



since calving, gave 13 per cent, more milk and 12 per cent, more butter 
than those scrubs which were brought to the station as mature cows. 
The numbers are not large, but the test as far as each individual was 
concerned was fair and extensive, covering for the most part four lactation 
periods. Since the animals while under test were given the same kind 
of treatment, we may justly conclude that the unfavorable development 
of those scrubs which were brought to the station as mature cows had 
permanently lowered their milk producing capacity. This is by no 
means a surprising conclusion ; on the contrary, it is exactly what would 
have been expected. The same relations obtain in all other characters 
in domestic animals; modifiability in its effect may be either permanent 
or transient. 



















1 




60 












h 


! 
/ 
/ 
/ 
/ 


N 


\.' 


50 












1 \ 

1 


pr^ 


/ 


X 


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kj 


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1 

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;r 30 
























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1 

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Year .ag.'oo .^^.^^ .oi:o2 ,^,,^^ •03^04 .j,^, ,,5 05-06 .oq-o/OT-'OS .03.09 'OO'IO .loiu'^l'l^ '12-'13'13-'U ,^^,.^^ 

Fig. 185. — A graphic representation of the results of breeding for high winter egg 
production. The period from 1899 to 1907, that of mass selection: 1908 to 1915 of geno- 
typic selection; the dotted line for low production, the broken line for high production. 
{After Pearl.) 



Modifiability and Breeding Value. — Often modifiability is of value 
to the herdsman in selecting his breeding stock, because it enables him 
to magnify differences between different individuals, and, therefore, to 
select his animals more accurately. For if modifiability is proportional 
to genetic variability, the increased production under forced conditions 
should be merely a somatic expression of the genetic potentialities of the 
individual. The trouble, however, is that variabihty of this kind is 
very often erratic in character, so much so that forced production, or 
even performance under more normal conditions, is often a very unsafe 
guide, except when reinforced by a knowledge of family history. 

A most striking case is reported from the Maine Station where breed- 



VARIATION IN DOMESTIC ANIMALS 



457 



ing investigations in egg production have been carried on for about 
20 years. An attempt was made during the period from 1899 to 
1907 to increase egg production by a system of mass selection. For 
breeding stock, pullets were selected which had produced 160 or more 
eggs during the first year of production, and they were mated to males 
the mothers of which had produced 200 or more eggs in their first year of 
production. The results of this rigid system of selection are shown 
graphically in Fig. 185. The soHd line connecting the dots represents the 
line of actual average winter egg production of the entire flock during 
the period from 1899 to 1915. The fitted straight line for the period 
1899 to 1907 shows clearly that during this period of mass selection there 
was an actual decrease, rather than an increase, in average winter egg 
production. Pearl and Surface point out that certain environmental 
factors may have had something to do with this decrease in production, 
but even when such factors are accorded a fair maximum effect, the 
evidence favors the interpretation that this system of mass selection 
has had absolutely no effect upon the average winter production of eggs. 
That this conclusion is a sound one is also supported by correlation 
studies of egg production of mothers and daughters in these flocks. In 
Table LIX the winter egg productions of 192 daughters are entered 

Table LIX. — Correlation between Daughters and Mothers with Respect 

TO Winter Egg Production, r = — 0.068 ± 0.048 {Data of Pearl and Surface) 

Mother's Egg Production 



Daughter's egg production 



0-3 
4-7 
8-11 
12-15 
16-19 
20-23 
24-27 
28-31 
32-35 
36-39 
40-43 
44-47 
48-51 
52-55 
56-59 
60-63 
64-67 
68-71 
72-75 
76-79 



Totals. 



12 



12 



10 



11 



20 



14 



11 



19 



32 



34 



1 
1 
1 

1 I 1 



52 
33 
21 

18 
14 
10 



6 
4 
1 
5 
4 
2 
6 
1 



1 

192 



458 



GENETICS IN RELATION TO AGRICULTURE 



against the productions of their mothers. The correlation of mothers and 
daughters with respect to winter egg production, as determined from this 
table, has the value, — 0.068 ± 0.48. There is, therefore, no demonstra- 
bly significant correlation between mother and daughter with respect 
to winter egg production, and so hkewise with spring and total egg pro- 
duction, the correlation coefficients do not differ sensibly from zero. 
Fortunately, however, for the efficacy of pure selection in breeding 
operations, correlation coefficients as low as this one are not usually 
reported. As an example of the more common condition, we give in 
Table LX the data of Rietz on correlation between mother and 
daughter with respect to milk production. The data are taken from the 
advanced registry records of the Holstein-Friesian cattle. They repre- 
sent, therefore, selected individuals, for both mothers and daughters have 
met the requirements for advanced registration. For that reason the 
values of the constants calculated from these data can only be regarded 
as approximately representing those which might be given in a population 
both of individuals which failed and those which succeeded in living up 
to advanced registry requirements. The value of the correlation coef- 
ficient in this case, 0.284 + 0.025, although low, indicates the existence 
of a significant degree of correlation. 

Table LX. — Cokrelation of Holstein-Freisian Mothers and Daughters 

WITH Respect to Production of Butter Fat, r = 0.284 + 0.025. 

Records for 7 Days {Data of Rietz) 

Production of Mothers 



Produc- 
































tion of 
daugh- 
ters 


11 


12 


13 


14 


15 1 


6 17 


18 


19 


20 


21 


22 


23 


24 


25 


Totals 


11 


1 


3 


1 


3 


2 


1 1 


1 




1 












14 


12 


1 


8 


13 


12 


6 


9 5 




3 


1 






1 










59 


13 


3 


18 


IS 


8 


13 1 


1 6 


3 


3 


1 
















84 


14 




14 


15 


i 14 


15 1 


1 12 


8 


2 


4 


1 
















99 


15 




12 


le 


16 


11 1 


1 8 


3 


3 


7 


3 
















90 


16 


1 


8 


IS 


16 


18. ] 


1 8 


2 


2 


5 


6 


2 




I 










93 


17 




3 


e 


) 11 


13 


4 8 


3 


4 


6 


4 


1 














63 


18 




3 


c 


) 2 


8 


5 5 


2 


5 


3 


2 


1 














41 


19 






r 


I 1 


4 


3 6 


2 


2 




1 


2 












L 


24 


20 






] 


7 


3 


3 3 




4 




1 
















23 


21 






] 


L 2 




1 






1 




1 














6 


' 22 




1 












1 




















2 


23 








1 


1 






1 




1 


2 














6 


24 








1 






1 






















2 


25 












1 
























1 


26 







































27 







































28 


















1 


















1 


Totals. 


7 


70 


9^ 


I 94 


94 ' 


ro 63 


25 


30 


30 


19 


9 


2 





1 


608 



VARIATION IN DOMESTIC ANIMALS 459 

Modifiability and Correlation. — ^Low correlation docs not necessarily 
indicate the relative degree and importance of modifiability in a popula- 
tion, for it is possible to have low correlation bctwecMi parent and off- 
spring as a consequence of genetic variability. This matter may be 
pointed our very clearly by considering a few abstract cases. Thus a 
population consisting largely of different, but homozygous, forms would 
exhibit a high degree of correlation between parent and offspring; 
whereas one containing more heterozygous individuals would display 
a lower correlation coefficient, simply because of the segregation which 
would take place in such a population. This, of course, is simply a de- 
velopment of the general case that an individual of genotype AA would 
produce only A individuals, provided A were completely dominant, 
whereas the genotype Aa would produce individuals of its own type and 
also of type a. On the other hand, modifiability may be the factor 
determining the value of the coefficient of correlation, even when the 
degree of conformance to a given standard is very high. Thus if a 
population be homogeneous genetically, as is the case in pure lines, then 
the correlation coefficient within the population would be nil, but when 
a number of different pure lines are mixed together and the correlation 
coefficient is determined for the mixed population, the value would be 
very high, although each pure fine within itself would exhibit zero 
correlation. 

This matter requires specific attention because some little confusion 
has grown up from the use of the coefficient of correlation as a measure of 
the intensity of inheritance, a practice of doubtful scientific propriety 
and one which might well favor misleading conclusions. The lack of 
correlation, for instance, in egg production of hens and their daughters 
theoretically does not indicate that attempts to increase egg production 
will be absolutely futile. It does indicate that some method of 
breeding must be adopted that will discount at their proper value the 
influence both of modifiability and genetic variability consequent upon 
segregation. The theoretical interpretation in this case is borne out 
strikingly by the practical results of the application of the method of 
genotypic selection, for as is shown graphically in Fig. 185, there has 
been a striking increase in egg production from the year in which the 
method of })recding was changed. 

Variation by Recombination. — Unquestionably the greatest possi- 
bility for improvement in animal breeding as well as in plant breeding 
lies in the isolation of recombinations of germinal elements which are 
better adapted to specific purposes. There is every reason to beheve, 
as we shall point out in the next following chapter, that the architecture 
of the germ-plasm, if we may use such a term, in domestic animals is 
similar to that in Di'osophila, that is, that the elements of the germ-plasm 



460 GENETICS IN RELATION TO AGRICULTURE 

are contained in definite arrangement in a fixed number of chromosomes. 
So far as our knowledge goes the chromosome mechanism behaves 
in a perfectly typical fashion. It is possible, therefore, to state with 
some degree of confidence that the mechanical details of recombination 
are the same in the higher animals as in the lower — ^the conception is a 
universal one. 

But it is perhaps necessary to enquire whence come the germinal 
elements which are the basis of the great diversity of characters exhibited 
by domestic animals. All breeds of the northern cattle are interf ertile ; 
they appear to belong to a common, related group ; and not only that but 
there are evidences of relationship of this great group with that other 
great group of humped cattle of the Orient, the zebus. Diversity among 
horses and sheep is not less striking than among cattle, and even in 
swine it is very great. So far as present evidence indicates most of 
this diversity is a consequence of polyphyletic origin; for from the be- 
ginning of domestication, man has constantly taken his livestock with 
him in his wanderings, and has allowed them to mix with whatever other 
types of the same species they might come into contact. 

Although a unanimity of opinion by no means obtains as to the 
path of descent of modern horses, the evidence of some kind of poly- 
phyletic origin may be regarded as conclusive. Ewart, who has given 
a good deal of attention to the problem of the origin of domesticated 
animals, inclines to the belief that the modern horse has sprung from the 
intermingling of several wild species which may have been connected 
with the three-toed horse of the Miocene period by different lines of 
descent. These wild species may be broadly characterized by their 
adaptability to different habits of life as horses of the forest type, of 
the plateau type, of the steppe type, and of the Siwalik type. Horses 
of the forest type were for the most part small, probably of a fawn 
color, and richly, although not conspicuously, striped. They were 
adapted to life in the forests and had very definite characteristics which 
are still seen in some of the modern breeds of horses. Fairly good repre- 
sentatives of this type are met with among the ponies of Scotland and 
Iceland, and other Eurasian reg ons. Evidences of forest ancestry are 
indicated in some modern breeds such as some Arabians and many of 
the modern breeds of draft horses such as the Suffolk, and in the 
Shire, Clydesdale, and Percheron to a certain extent. The plateau 
type of horse comprises many different races, but in general they 
are all finely built, slender limbed, fleet ponies of which modern 
representatives are the Celtic ponies of the British Isles and Mexican 
ponies. Arabs and Barbs and through them the modern Thoroughbred 
are largely of plateau ancestry; and there is considerable evidence of 
the same blood in some Shetland ponies. The steppe type of horse, 



VARIATION IN DOMESTIC ANIMALS 461 

now represented by the Mongolian or Prejvalsky horse, was a fleet, 
active horse of about 13 hands height, perhaps most distinctively 
characterized by its large head and Roman nose. The modern Shire and 
Clydesdale exh'bit this type of head, and evidently trace along one line 
to steppe ancestors, as do other Roman-nosed breeds. The Siwalik horse 
of India was a tall finely built horse of racing type. Horses of this type 
may have been over 15 hands high, and they had a peculiar promi- 
nence between the eyes which is still met with among some Arabian and 
Indian horses. Some Thoroughbreds today exhibit characters which 
connect them with ancestors of the Siwalik type. The evidence, there- 
fore, is very strongly in support of the belief in the polyphyletic origin 
of modern breeds of horses. In the characters of these ancient ancestors 
of the horse we may find represented practically all the characters of 
modern horses. 

In historic times there is abundant evidence that mixing of distinct 
types of horses was a very common practice. In Europe the fleet, finely 
built horses of the Oriental desert type, particularly the Arab and the 
Barb, have been used freely in perfecting practically all modern breeds 
of horses. At about the beginning of the 18th Centurj^, three Ori- 
ental stalhons, the Godolphin Barb, the Byerly Turk, and the Darley 
Arabian were used extensively in England, and from this foundation 
stock sprang the Thoroughbred and Hackney, and later in America the 
Standard bred. As late as 1820 two gray Arabian horses Godolphin and 
Gallipoli were used on draft mares in La Perche, and they had a remark- 
able influence in the direction of superior quality and action in Percheron 
horses. And this is only one side of the story of diversity in the founda- 
tion stock of modern breeds of horses, for without exception thej^ all have 
a comparatively short history of strict matings confined to the breed 
standards. 

The horse has been chosen merely as an example; other kinds of 
livestock show just as striking ancestral diversity. Among cattle there 
is evidence of zebu ancestry in some breeds like the Shorthorn, whereas 
the Aberdeen-Angus seems to trace to an ancient Syrian race. As in 
horses so in cattle there has been much mixing of types within historical 
times. In the case of the domestic fowl, the opinion is usually defended 
that there is evidence of monophyletic origin, the wild jungle fowl, 
Gallus bankiva being regarded as the common ancestor. But there is 
evidence that the Malay breeds have descended from another species, 
and in view of the freedom with which the Malay breeds cross with other 
breeds of fowl, it may be wise to reserve judgment of the monophjdetic 
origin of barnyard fowls. It is, however, proper to state that many of 
the breeds of fowls do show differences which are of the value of simple 
factor differences, or recombinations of a few such original differences; 



462 GENETICS IN RELATION TO AGRICULTURE 

but some characters seem to be too complex to admit readily of such form- 
ulation. We may, therefore, justly draw the general conclusion that the 
polyphyletic origin of modern breeds of farm animals has been a fruitful 
source of germinal diversity. 

Mutation in Domestic Animals. — ^The occurrence of mutations in higher 
animals appears to be extremely rare, at least cases concerning which 
definite evidence exists are very few. Those, however, which have 
occurred appear to be strictly analogous in their nature and hereditary 
behavior to the factor mutations in Drosophila; they involve changes in 
definite loci in the germinal material. 

The Ancon sheep, one of the earliest authentic cases of the occurrence 
of a mutation, has been discussed at some length by Darwin. This type 
of sheep first appeared in a small flock kept by a Massachusetts farmer, 
Seth Wright. The mutant, a ram lamb, was dropped in 1791. It had 
an unusually long back, and short crooked legs, characters which appealed 
to Farmer Wright, because sheep which possessed them could not readily 
leap the fences which were so laboriously constructed at that time. 
Seth Wright set to work, therefore, to establish a flock of Ancon sheep, 
and he had no difficulty in doing so. Humphreys, commenting upon the 
case, emphasizes the trueness with which the Ancon sheep bred to type, 
there being only one doubtful case of a mating of an Ancon sheep and 
ram which produced anything but Ancon offspring. In segregation the 
character was always sharply discontinuous. The evidence that the 
Ancon sheep arose by mutation is not unimpeachable, it might have 
arisen by normal segregation of a recessive factor; but since no other case 
of such segregation of the Ancon factor has been observed, the mutation 
hypothesis appears best to account for it. The Ancon breed of sheep is 
now extinct, having been displaced e'ntirely by Merinos. 

Darwin discusses another case of a sport in sheep, namely that from 
which the Mauchamp breed of France is derived. In this case a Merino 
lamb which was dropped in 1828 had a fleece much superior to that of 
the general flock; the wool was long and silky, and so desirable as to 
command a price 25 per cent, greater than that of the best Merino wool. 
By judicious use of this ram and subsequent rigorous selection M. Graux 
was able to establish on Mauchamp farm a breed of sheep having all the 
superior fleece qualities of this animal. 

But the case was not clear cut and deflnite like that of the Ancon 
sheep. There was no evidence of distinct alternative inheritance, and 
rigid selection was necessary in order to establish the new characters in a 
pure race. In fact reexamination of the records in this case shows 
clearly that the original ram lamb was not a mutant, but merely an 
accidental hybrid from a ewe of the Mauchamp flock and a Dishley 
ram of an adjoining flock. This case, therefore, as Nathusius points 



VARIATION IN DOMESTIC ANIMALS 463 

out must be struck from the records of observed mutations in domestic 
animals. 

Another case in sheep which has no importance, perhaps, from a 
practical standpoint is reported from Norway. Wriedt writes that 
among the old short-tailed sheep of certain districts in Norway, individuals 
are found occasionally which possess very short ears. The short-ear 
character is dominant and alternative in expression when contrasted 
with the normal long-ear type. The records of one rather small flock 
descended from a single short-eared ewe showed that during 20 years 
not a single case of the production of short-eared offspring l)y long-cared 
parents was observed. The fact of the simple character difference between 
long and short ears is clearly established by these records, but whether the 
short-ear type is the result of relatively frequent mutation may be a 
matter of some doubt. Ritzman in fact has pointed out that the short- 
ear character of native ewes when contrasted with the normal long-ear 
type of such breeds as the Rambouillet, Southdown, and Shropshire 
behaves as a simple character which exhibits alternative inheritance. 
This confirms Wriedt's observation as to inheritance, but does not add 
any evidence as to the origin of the short-ear type. 

The most frequently cited evidence of mutation in domestic cattle 
is that of the polled character. Although this character is a simple 
dominant the evidence of mutation or segregation in particular cases is 
not always clear. Polled cattle have been known from ancient times and 
they have not been entirely wanting in the foundation stock of any 
modern breed. Within recent years the Polled Hereford and Polled 
Durham breeds have been established by utilization of polled mutants 
and by grading from horned Hereford and Shorthorn cattle respectively. 
Within these breeds the so-called double standard animals, i.e., those 
which are eligible to registry in the corresponding horned breed herdbooks 
as well as in the polled records, presumably have all sprung from definite 
mutations. However, of nine polled sports Ksted by Spillman all except 
one were known to have near relatives that were polled. The two 
Hereford bulls Wilson 126,523 and Variation 152,699 both apparently 
came from horned ancestors. They were used by Boj^d in establishing 
a pure race of polled Herefords. They were both heterozygous, for in 
matings with horned Herefords approximately half the offspring were 
polled and half horned. A herd of pure-bred polled Holstein-Friesian 
cattle has, also, been established in this country, but it appears to have 
been established by utilization of normal polled animals of which there 
are some representatives within the breed. Whatever the explanation 
as to the origin of these rather numerous polled sports, however, there is 
no question as to the correctness of Bateson's suggestion that the polled 
condition is dependent upon a single dominant factor difference from 



464 GENETICS IN RELATION TO AGRICULTURE 

the horned condition; so that, given a polled individual, it is the simplest 
kind of a Mendelian problem to establish a race all the individuals of 
which are polled. 

From the more strictly economic side there is very slight evidence of 
the occurrence under observation of any definite mutation. Arenander 
reported some results with milk cows which might indicate mutation. 
The evidence deals with a cow Ortvart and her progeny. This cow 
gave milk of an abnormally low fat content, and she transmitted this 
character to her daughters. But nothing definite is known as to the 
parentage of Ortvart. Furthermore she was as strikingly distinct in 
color characters as in low fat content of the milk; she was white with 
black ears and small spots. Of her seven daughters five are described 
as of the same color and pattern, the remaining two were of the same 
pattern but the ears and spots were red. There is no acceptable evidence 
of mutation in this case, but the evidence of inheritance of the trait of 
producing milk of low fat content is unmistakable. So also Pearl's 
case of a mutant in egg production is, probably, as he himself states, 
merely a case of extreme Mendehan segregation. The data for this 
case are given in Chapter XXXI. We may conclude, therefore, and 
rightly, that any system of herd improvement founded on the search 
for and utilization of mutants is doomed from the beginning to failure, 
for mutants of a beneficial character appear so rarely as to have almost 
no practical significance. If by some fortunate chance a breeder should 
find himself in possession of a favorable mutant individual, however, 
it is a simple problem in Mendelism to estabHsh its characters in a 
constant race. 



CHAPTER XXIX 
MENDELISM IN DOMESTIC ANIMALS 

Although there is a lamentable dearth of specific cases of Mendelian 
inheritance in domestic animals, there is evidence enough to indicate that 
Mendehan principles are of general validity. The difficulty is merely 
a practical one consequent upon the long time and great financial ex- 
penditure which are necessary for collecting critical data in animals. 
Thus far we may state confidently, however, that none of the known facts 
of heredity in farm animals, or in man himself for that matter, is in 
conflict with Mendehan interpretation. Such an interpretation cannot, 
however, be applied satisfactorily until more detailed knowledge has been 
collected of the relation of various characters to one another. Thus far 
practically all Mendehan data in farm animals are from herdbook 
records, and we have gone about as far as it is possible to go with such 
material. Henceforth it will be necessary to depend almost entirely 
upon experimental breeding, if any progress is to be made. This chapter 
is designed to give a record of about the present status of our knowledge 
of Mendelian heredity in farm animals. 

Mendelism in Horses. — ^Practically all the Mendehan data for the 
horse thus far collected deal with coat color. About the only additional 
data we have is that for the trotting character as opposed to pacing in the 
Standard bred. However, in addition to the characters just mentioned, 
Hurst lists the following contrasted characters as allelomorphic: concave 
and convex faces; straight and curved thighs and hocks; prick-ear, 
drooping ear, forward droop and outward droop of ears; sprinters and 
stayers; liability to cataract blindness, breaking blood-vessels and para- 
lytic roaring (contrasted with normal conditions) ; long-back and short- 
back. The natural trotting gait appears to be a simple dominant to 
pacing, although there is still considerable doubt as to these characters. 
The records in Table LXIV are of interest in this connection for they 
show that of these ten stalhons only one, Electioneer 125, was 
homozygous for the trotting character. It is true that two pacers 
are credited to him, but this may possibly be accounted for by training, 
for it is often possible to change the gait of a horse by proper attention. 

Of coat colors, chestnut appears to be the simplest one. Chestnut 
includes a series of colors varying in depth from dark liver to light sorrel. 
Chestnut mated to chestnut produces only chestnut. Of 14,131 matings 
30 465 



466 GENETICS IN RELATION TO AGRICULTURE 

tabulated by Anderson only sixteen of other colors were recorded. An- 
derson is confident that these were all mistakes, and since herdbook records 
are in error to the extent of about 2 per cent, this explanation would 
appear to be acceptable. Suffolk Punch horses are always chestnut, 
but even in other breeds when chestnuts are produced from matings of 
horses of other colors, they, when mated inter se, produce only chestnut 
progeny. Of the shades of chestnut, sorrel appears to be recessive to 
chestnut proper and distinct from it, according to data collected by Mc- 
Cann from studbook records. According to Wentworth, lighter mane 
and tail, often met with in sorrels, is recessive to the normal darker 
coloration. 

Black is a simple dominant to chestnut, but the data in this case are 
not so clear cut as for chestnut. For this analysis, we represent the 
black factor by C. Black varies in shade from a deep, clear black to 
seal-brown, but very little is known about the relations of the different 
shades to one another. 

Bay is black with a dominant restriction factor B which confines 
the expression of black to the mane and tail and the extremities in general, 
the rest of the body being covered with bay hairs. Since the factor B 
acts only on C it may be present in a latent condition in chestnut horses. 
This conception, originally suggested by Wentworth, differs from other 
hypotheses in that it accounts for the fact that black X chestnut matings 
give a high proportion of bay offspring. Castle suggests that chestnut 
horses carrying the bay factor be called sorrel and those lacking it chest- 
nut; but the data presented by McCann indicate that the terms should 
be reversed, if it be desired to bring the terminology into conformance 
with common practice. 

The position of the so-called brown horses in the Mendelian scheme 
is a matter over which there has been much speculation. Seal-brown 
appears to be merely a shade of black; but mahogany-brown, i.e., bay with 
black patches alternating with bay on the sides of the body, seems to be 
bay heterozygous for B. The data of Table LXI may be explained by 
such a formulation, but actual experimental investigations should be 
carried out, if it be desirable to determine the relations accurately. 

Gray is a color in which black and white hairs are intermingled in the 
coat. Gray foals at birth are very dark, but with age they become pro- 
gressively lighter until in old horses the color is almost white. As a color 
gray is not much favored in any except the Percheron breed. The gray 
factor G^ is a dominant factor, and its relations to those which have just 
been mentioned are such that when it is present the coat color is gray 
irrespective of which of the other factors may also be present. In the 
Clydesdale gray is tabooed, consequently all gray stallions are castrated, 
and gray mares are bred to stallions of a different color. According to 



MEN DELI SM IN DOMESTIC ANIMALS 



467 



Table LXI.^The Transmission of Coat Colok in Horses in Various 

Matings 



Mating 



Chestnut X Chestnut. 
Chestnut X Black .... 
Chestnut X Brown . . . 

Chestnut X Bay 

Black X Black 

Black X Brown 

Black X Bay 

Brown X Brown 

Bay X Brown 

Bay X Bay 

Roan X Chestnut .... 

Roan X Black 

Roan X Brown 

Roan X Bay 

Roan X Gray 

Roan X Roan 

Gray X not Gray 

Gray X Gray 



Chestnut 



14,115 

111 

60 

597 

11 

14 

123 

13 

177 

474 

9 

1 

1 

9 



Black 



10 

83 

32 

56 

295 

198 

295 

64 

132 

107 

3 

11 

5 

5 



Not gray 
Not gray 



Brown 



1 

20 

31 

49 

15 

219 

261 

334 

817 

300 

2 

3 

16 

13 

3 



Bay 



5 

124 

130 

764 

5 

115 

634 

157 

1,449 

2,831 

9 

1 

18 

39 

3 

528 
18 



Gray 



1 

1 
1 
5 
2 
439 
47 



Roan 



14 
15 

28 

50 

7 

9 



Went worth, Cole found in tabulating the offspring of gray mares re- 
corded in the Clydesdale studbook that exactly 50 per cent, were gray 
and 50 per cent, not gray, which is in strict conformance to expectation. 
Roan is a coat color characterized by a slight sprinkling of white hairs 
in a pigmented coat. The roan color is even less popular in breeds than 
gray, and it occurs to any great extent only in Belgian draft horses. The 
roan factor, R, is a dominant pattern factor independent of any of the 
color factors. It is, therefore, possible to have gray roans, red roans, 
blue roans, and chestnut roans according as the pigmented coat color is 
gray, bay, black, or chestnut, respectively. Gray roans are not dis- 
tinguishable from ordinary gray of course, except at birth, so that this 
class is merely a genetic one not recognized in practical breeding opera- 
tions. At birth gray foals are black, whereas gray-roans are black 
with interspersed white hairs. The data of Table LXI indicate plainly 
enough that roan is a dominant color, but some reports of individual 
animals are even more interesting. Thus J. Wilson reports that a red- 
roan Belgian stallion standing for service in Story County, Iowa, was 
bred to all classes of mares, but all of the 256 foals he sired were 
red-roan like himself. Another red-roan stallion sired 254 colts of which 
230 were red-roan and the remaining 24 blue-roan. These two stallions 
must have been homozygous for the roan factor, and their breeding re- 
cords establish clearly the dominance of the roan coat color pattern. 



468 GENETICS IN RELATION TO AGRICULTURE 

This completes the formulation, so far as our present knowledge goes, 
for the series of colors, usually met with among horses, but there are a few 
others more rare and less in favor, the position of which from a genetic 
standpoint is almost wholly speculative. There appears to be, however, 
a dominant dilution factor, /, which acts upon all the color factors. 
According to Wentworth black with this factor becomes mouse colored, 
bay becomes dun of the particular shade known as buckskin, chestnut 
becomes yellowish dun, and sorrel with lighter mane and tail becomes 
cream colored with lighter mane and tail. The evidence, however, is 
by no means extensive enough to be conclusive and should receive 




Fig. 1S6. — The skewbald Iceland pony, Tundra, her skewbald filly, Circus Girl, by a 
bay Shetland pony, and her hybrid foal, Sir John, by the Burchell zebra, Matopo. {After 
Ewart.) 

experimental verification. There seems, also, to be a white which is 
distinct from the faded gray of old horses. This white is dominant to any 
color. Castle considers it an extreme extension toward white of the 
spotted condition. The types of spotting are mostly dominant over 
uniform coloration, and often the pattern is very faithfully reproduced. 
This statement apphes to stars, blazes, skewbald markings, calico types 
of pattern, and other kinds of white spotting of the same general type. 
Fig. 186 shows a case of accurate reproduction of skewbald markings by 
the offspring of an Iceland pony when bred to a bay Shetland pony 
stallion. Tundra had previously produced a dun foal to the service of a 
stallion of unknown coat color, and subsequently she produced another 
skewbald foal. Her zebra hybrid foal, however, was of a dun color, 



MENDELISM IN DOMESTIC ANIMALS 



469 



indistinctly striped. Indistinct striping and other types of marking 
occasionally occur in horses, but the hereditary relations concerned in 
their appearance are not well understood. 

J. Wilson has advanced a formulation which is fundamentally different 
from the one which has been outlined above. He assumes that gray, dun, 
bay, black, and chestnut form a series of polygamous factors as he calls 
them, multiple allelomorphs according to our terminology, in which each 





g" 


g' 


0" 


g' 


G 


G 


Gg' 
Gray 
3 gray : 1 chestnut 


Gg'^ 
Gray 
3 gray : 1 black 


Gg" Gg^ , GG 
Gray Gray j Gray 
3 gray : 1 bay 3 gray : 1 dun ' All gray 


g' 


g-'g' 

Dun 
3 dun : 1 chestnut 


g'^g'' 

Dun 
3 dun : 1 black 


Dun 
3 dun : 1 bay 


g'^g'^' 

Dun 

All dun 




^ 


g^g" 

Bay 

3 bay : 1 chestnut 


Bay 

3 bay : 1 black 


Bay 

All bay 






g' 


Black 
3 black : 1 chestnut 


g^g'' 

Black 
All black 






g" 


g'g' 

Chestnut 
All chestnut 







Fig. 187. — The genotypes of gray, dun, bay, black, and chestnut coat colors in the 
horse according to the formulation of Wilson. The way in which animals of a particular 
genotype behave in a subsequent generation when mated together is shown in each square. 

member of the series in the order named is dominant to succeeding 
members and recessive to the preceding ones. In Fig. 187 we have out- 
lined the possible combinations which could occur within such a series of 
multiple allelomorphs, and the consequences of such combinations. In 
addition to this pentuple series of multiple allelomorphs, Wilson assumes 
that there is an independent dominant roan factor for the roan pattern, 
and that there are modifying factors which affect the shade and distribu- 
tion of pigment. Brown he considers a modified bay. Although this 
formulation is undeniably simpler than the one which was discussed first, 
it is highly probable that this simplicity is very misleading. The series 
of colors evidentl)^ does not well conform to the general rule of multiple 
allelomorphism of exhibiting a graded series with a diminishing in- 
tensity; but, so far as the stud book records of Table LXI are concerned, 
about the only place where this formulation fails to meet all the ob- 



470 GENETICS IN RELATION TO AGRICULTURE 

served facts is in the relation of bay, black, and chestnut to one another. 
Wilson's formulation does not account for the production of bay foals from 
matings of chestnut and black, although such matings produce a large 
proportion of bay foals, whereas the first formulation accounts for them 
very simply. Moreover the first analysis is more nearly in harmony with 
our knowledge of the inheritance of coat color in rodents, which is so 
well understood that there is no question of this kind as to factor relations. 
Wilson's formulation, however, is of this service: it points out clearly 
how uncertain are analyses based on herd book records, and thereby at 
the same time indicates the need of actual experimental investigation. 
It would be a very simple matter to demonstrate experimentally which 
of these analyses accounts for the actual factor interrelations. 

We cannot refrain here from indicating some of the consequences in 
breeding practice of a formulation such as the one we have favored in 
this discussion. The really important feature of the analysis, of course, 
lies in the emphasis it gives to the definiteness of phenomena of coat color 
inheritance in the horse. In every case there is a definite reason why a 
horse should be of a certain color, and the reason is comparatively 
simple. Moreover, since the phenomena are so definitely predetermined, 
it is possible within certain limits to control them. 

To take a definite instance, the government has set itself the task at 
the Iowa station of creating a gray breed of draft horses. Since gray is 
dominant to all the common horse colors save roan, it is impossible to get 
gray from matings of other colors. Moreover, grays when mated to- 
gether produce gray, bay, brown, black, and chestnut foals, according 
to the particular gray genotypes which are involved. Grays of the 
genetic constitution HhBbGg mated inter se produce the entire series of 
colors in the ratio 

48 gray: 9 bay and brown: 3 black: 4 chestnut. 

The bay, brown, black, and chestnut offspring of such matings, or 
any other for that matter, might be mated together ever so often, yet 
they would never produce gray foals, although themselves the offspring 
of gray horses. 

Accordingly the method which should be followed in estabhshing a 
gray breed of draft horses is not difficult to map out. An effort should be 
made to get homozygous gray horses for breeding stock. Such horses, 
of course, will be met with only in breeds in which there is no prejudice 
against gray, as for example in the Percheron breed; but, if it should be 
thought desirable to utilize some of the good qualities of other breeds in 
which gray is not a favored color that may be done at the sacrifice of 
uniformity of color in the first generations. Matings should, however, 
always be of gray to gray individuals, and all animals of other colors should 



MEN DELI SM IN DOMESTIC ANIMALS 471 

be disposed of. As the type becomes well establislied iind there is a wide 
field of choice among the grays the selection should be further refined by 
disposing of botJi parents of any foal of a color other than gra3\ Bj^ this 
method it should be a simple matter to establish a gray breed of draft 
horses which would not only breed true to color among themselves, but 
would also give only gray foals when mated to bay, brown, black, or 
chestnut horses. 

Mendelism in cattle. The state of knowledge of the inheritance of 
coat color in cattle is even less satisfactory than that in hoi-ses, but there 
are some results from experimental investigations which have firmly 
grounded our knowledge of certain points of it. 

White is a natural starting point in the discussion of Mendclian 
inheritance of coat color in cattle, but it proves to be by no means capable 
of simple treatment. The difficulty of dealing with white in cattle appears 
to be due to the variety of whites of different genotypes which are met 
with. Thus the white of Shorthorn cattle is apparently never a true 
white, for the eyelashes, face bristles, and particularly the ears always 
bear some red hairs, although often so few that they are ordinarily over- 
looked by breeders. This type of pigmentation is not an extreme con- 
dition of red and white blotching which is often met with in Shorthorn 
cattle, but it is entirely independent of spotting. This is indicated by 
the clean cut segregation which this type of coloration exhibits. 
Lloyd-Jones and Evvard present data which favors the explanation that 
white with colored extremities depends on a recessive extension factor, e. 
Another type of white met with in modern breeds appears to be merely 
an extreme condition of spotting. Such whites are not uncommon 
among Ayrshire and Holstein-Fresian cattle, but they are undoubtedly 
genetically different from whites with colored extremities. The wild 
white Park cattle of Britain and the white feral herds mentioned by 
Darwin always have colored ears, but it is doubtful whether they are 
genetically identical with the white Shorthorn. They apparently 
produce colored calves at times in spite of the fact that such calves are 
never retained in the breeding herds. It is known that some of these 
white herds were of mixed origin, and that some of them are creamj^ 
white rather than pure white. Accordingly it is not impossible that the 
colored calves which arc produced by wild Park cattle are the result of 
recombinations of complementary factors rather than of recessive 
segregation. The problem of white in cattle is far from a complete 
solution. 

Red in cattle varies from a very dark red to light yellowish red. In 
the early days of the Shorthorn breed all these shades were represented 
but now the very dark reds and the yellowish reds are looked upon with 
disfavor. There is a httle evidence of sharp segregation between some 



472 



GENETICS IN RELATION TO AGRICULTURE 



shades, but on the whole our knowledge of the relations of the different 
shades to one another is very imperfect. In Shorthorn cattle red appears 
to breed true, at least records of white calves from red X red matings are 
so rare as to lead one to suspect they were due to error of registration. 
Compared with white in this breed red represents a condition of extended 
pigmentation, dependent upon the dominant factor, E. Here a difficulty 
is introduced by the fact that red X white matings ordinarily produce 
roan rather than red and white offspring. 




Fig. 188.— a white polled heifer with black ears and muzzle; an Fi individual from the 
cross Galloway X white Shorthorn. (After Lloyd-Jones and Evvard.) • 

The roan color in cattle, like roan in horses, appears to depend upon a 
definite dominant factor, R. Strictly of course roan is not a color, but a 
pattern effect due to admixture of white hairs in a pigmented coat, and it 
may affect black as well as red. This roan type of coloration is character- 
istic of Shorthorn cattle. The predominance of roan animals in this breed 
probably accounts for the fact that white mated to red usually gives roan 
offspring, for the whites are derived almost wholly from roan matings 
and they should therefore of necessity often bear the roan factor. There 
are also some apparently authentic accounts of red and white animals, 
the progeny of matings of red X white, such as the case of the noted white 
Shorthorn bull, Whitehall Sultan, which sired fifteen red calves out of 
various red cows. Were it not for this, J. Wilson's assumption that roan 



MENDELISM IN DOMESTIC ANIMALS 473 

is merely a heterozygous condition of red and white, or in terms of the 
above factors, the expression of the Ee genotype, would be satisfactory. 

The black color of Aberdeen-Angus and Galloway cattle is definitely 
dominant to red. This is shown by the fact that such cattle occasionally 
produce red calves. Since red was a common color in the early founda- 
tion stocks of the two breeds, it follows that the production of occasional 
red calves is merely a consequence of the handing down of the factor h in 
a heterozygous condition, and of the rare matings of animals both of which 
are heterozygous for it. In the black-and-white Dutch cattle of various 
types it is also not uncommon to have red-and-white offspring produced 
from black-and-white matings, but red-and-white matings never produce 
black-and-white calves. The factor B, as well as 6, is affected by the 
extension factor E. Lloyd-Jones and Evvard have demonstrated this 
fact in the Fi of Galloway X white Shorthorn matings, the white animals 
of which had black ears and muzzle like the Chillingham Park cattle. 
An excellent representation of such an animal is given in Fig. 188. Black 
is, also, affected by the roan factor in the same way as red, giving a 
blue-roan or blue-gray. The famous blue-gray cattle produced by 
mating Galloway, or less frequently Aberdeen-Angus, cows to white 
Shorthorn bulls are evidence of this fact; but the critical test necessary 
to decide between the two rival hypotheses of blue-gray as a simple 
heterozygote between black and white or as a consequence of the action 
of a separate roan factor has not yet been carried out. 

Of the relations of other colors to each other, we know very little. 
J. Wilson states that there are five colors in cattle which breed true, 
namely black, red, light dun, brown, and white, and that aside from 
matings of black and red which give blacks, matings of different colors give 
what may be called, for convenience, intermediates. The exact colors 
which represent the heterozygotes are given in Fig. 189. But there is no 
satisfactory way of determining the relations of the factors to each other 
save by the experimental test. Wilson presents some data gleaned from 
the Highland Cattle Herdbook on the relation between black, red, brindle, 
yellow, dun and light dun; but they are not of such a nature as to be capa- 
l)le of accurate Mendelian interpretation. The data do, however, indicate 
that black is the highest member of the series, and that red comes next to 
it. As to the relation between black and yellow, dun, light dun, and 
brown, we are still in some doubt in spite of positive statements by Wilson. 
On the one hand we have evidence that the foundation stock of Aberdeen- 
Angus cattle contained animals black, red, yellow, dun, light dun, 
brown-backed and other mixtures of these, only one of which, red, has 
survived as a simple recessive. This would lend support to Wilson's idea 
that the relation of blacks to the colors other than red is not that of 
simple dominance. Black, however, appears to be dominant to the fawn 



474 



GENETICS IN RELATION TO AGRICULTURE 



of Jersey, as is shown by the evidence from Jersey-Angus crosses reported 
by Kuhlman, the Fi of which is black. A Httle definite experimental 
evidence on these problems would be of more service than much 
speculation, 

J. Wilson arranges the entire series of coat colors in a system of multi- 
ple allelomorphs much like the one he advocates for the inheritance of coat 
color in horses. The essential features of this system are shown in Fig. 
189. This formulation, however, falls down in one important instance 



^d 



B 



Id 



Bb 


Bb^ 


Bb'' 


By 


BB 


Blue-roan 


Brindle 


Dun 


Black 


Black 


1 black 


1 black 


1 black 


3 black : 1 red 


All black 


2 blue-roan 


2 brindle 


2 dun 






1 white 


1 brown 


1 light dun 






b'b 


ft^b" 


¥b'' 


b'b' 




Roan 


Red brindle 


Yellow 


Red 




1 red 


1 red 


1 red 


All red 




2 roan 


2 red brindle 


2 yellow 






1 white 


1 brown 


1 light dun 






¥b 


b''b^ 


ftdftd 






? 


Light brindle 

1 light dun 

2 light brindle 
1 brown 


Light dun 
All light dun 






&»6 


b-b"- 






? 


Brown 

All brown 








bb 






White 










All white 











Fig. 189. — Wilson's interpretation of the inheritance of coat color in cattle, 
are B, black; f, red; 6'', light dun; b^, brown; and b, white. 



The factors 



in which we have definite evidence. Lloyd-Jones and Evvard report the 
production of some red animals in F2 in matings of Fi blue-gray animals 
from a cross between the black Galloway and the white Shorthorn. 
Such a result is entirely unprovided for in Wilson's scheme, and very 
probably other portions of it would break down before critical experi- 
mental tests. 

Of other characters in cattle, the white-face pattern of Hereford cattle 
is dominant to colored face. Fig. 190 shows a typical instance of this 
kind. This dominance of the white-face pattern extends to species 



MEN DELI 8M IN DOMESTIC ANIMALS 



475 



hybrids, for hy])rids between the Hereford and bison exhibit the typical 
white-face markings. The Fi hybrid between the zebu and Hereford, 
however, has a broken colored face, as shown in Fig. 206. The early 
history of the Hereford breed indicates that mottled-faced animals were 
not uncommon in the foundation stock, and today they are met with not 
infrequently in grade Hereford cattle. The characteristic pattern of 
Dutch belted cattle appears, also, to be a dominant character as con- 
trasted with self-coloration. Another color character, the black of 




Fig. 190. — California Favorite, grand champion steer at 1916 International Livestock 
Exposition. Out of a red Shorthorn cow by a Hereford bull. The Hereford pattern is 
completely dominant. {Photo from G . H. True.) 

Ayrshire cattle, exhibits sex-limited relations and will, therefore, be dis- 
cussed in another chapter. The polled character in cattle, as we have 
pointed out before, is clearly dominant to the horned condition, but the 
Fi may exhibit slight scurs. The breeding of cattle for the polled condi- 
tion is a simple problem in Mendelism involving a difference in a single 
pair of factors. The study of other characters in animals, particularly 
those which are of economic importance from a Mendelian standpoint, 
has just begun. 

Mendelism in Sheep. — As we have already noted the short-eared 
condition in sheep has been shown to be a simple dominant to the long- 
eared character. The factor for black wool in sheep is recessive, as 



476 GENETICS IN RELATION TO AGRICULTURE 

shown by the evidence of C. B. Davenport. Black sheep mated together 
produce only black sheep, and it is probable that the black sheep met 
with occasionally in white flocks represent the cropping out of 
homozygous recessives, hke the occasional red calves in some herds of 
black cattle. Wilson reports that black face and white face in sheep 
represent a simple character contrast, the F] heterozj'gote being gray. 
There are, however, breeds of sheep which have gray faces, so that it is a 
matter of question whether Wilson is not here again advocating too 
simple an explanation. The inheritance of horns in sheep has been 
subjected to MendeUan analysis, but questions of sex are involved in 
this case, for which reason treatment is reserved for a succeeding chapter. 

Mendelism in Swine. — About all that is known of Mendelian in- 
heritance in swine deals with coat color. White is dominant to colored, 
but segregation is rarely definite in F2, apparently because white often 
carries latent pattern factors which are responsible for the production of 
belted and variously spotted indi\'iduals in segregating populations. The 
belted pattern of Hampshire swine is dominant to uniformly colored coat, 
and it is clearly independent of the particular color in the coat. The 
e^ddence regarding spotting of the coat is conflicting, but at least one 
type appears to be dominant. Black coat color is dominant to red, in 
which respect it is like the corresponding color in horses and cattle. Red 
in hogs may be of different shades, and like chestnut in horses, there is 
evidence that the lighter shades are most easily maintained. There are 
reports in the literature of roan swine, but the genetic constitution of 
these is imperfectly known. As to other characters, the union of the 
two toes found in mule-footed hogs is dominant to the normal condition, 
and according to Spillman represents a single dominant factor difference 
from the normal condition. 

Mendelism in Poultry. — -The Mendelian inheritance of comb char- 
acter in fowls has been discussed in another chapter. The heterozy- 
gous constitution of the Blue Andalusian fowl has, also, been discussed in 
detail. Blue Andalusians mated inter se always produce Blue, White 
Splashed, and Black Andalusian fowls in the ratio of 2 : 1 : 1 ; but black 
birds mated with white-splashed ones produce nothing but Blue Anda- 
lusians. The breeder, therefore, must maintain pens of black and 
splashed white bu'ds if he desires to produce progenies made up wholly 
of Blue Andalusians. The fact that the Blue Andalusian never breeds 
true, however, should be sufficient justification for refusing to recognize 
it as a distinct breed. It is of interest to note that the Blue Breda bears 
the same relation to a black- and a white-splashed form as the Blue Anda- 
lusian. It is just, however, to note in passing, as Pearl in fact has pointed 
out, that there is some doubt of the authenticity of these cases. They 
require further investigation. 



MENDELISM IN DOMESTIC ANIMALS 



477 



The great variety of diverse characters in fowls, particularly in color 
and pattern of plumage, has made them unusually excellent subjects 
for Mendclian investigations. Although extensive observations have 
been made upon a large number of hybrids, however, the exact factor 
relations are not well understood for many characters. In the list in 
Table LXII some data are presented with respect to dominance of certain 
contrasted characters; but these contrasts should not be taken to in- 



Table LXII. — Results of Certain Contrasts in the Domestic Fowl 



Dominant 



Recessive 



Expression 



Barred plumage pattern 

Beard 

Black 

Black iris color 

Black 

Buff 

Booting 

Broodiness 

Brown striped down .... 

Crest (Silky) 

Dark shank color 

Extra toes 

Feathered shanks 

Frizzle feathering 

High fecundity 

High fecundity 

Joining of toes 

Leaf comb (Houdan) . . . 

Leghorn pattern 

Long tail (Japanese) 

Normal head 

Pea comb 

Plain feathering 

Plain heel 

Rapid feathering 

Red 

Red ear lobe 

Rose comb 

Rose comb 

Rumplessness 

Self -color (Minorca) .... 

Silky pigmentation 

Single comb 

Walnut comb 

WTiite (Leghorn) 

Yellow shank color 

Yellow skin color 



Uniform coloration 

No beard 

Red, buff 

Brown, red, pearl iris 

^Miite (Minorca) 

WTiite (Minorca) 

No booting 

Non-broodiness 

Pale brown down 

No crest 

Light shank color 

Normal foot 

Clean shanks 

Normal repent feathering 

Low fecundity 

Low fecunditj'' 

Normal foot 

Single comb 

White (Minorca) 

Normal tail 

Cerebral hernia (Polish) 

Single comb 

Silky feathering 

Vulture hock (Silky) 

Slow feathering 

WTiite (Minorca) 

WTiite ear lobe 

Leaf comb 

Single comb 

Normal uropj'^gium 

Hackle lacing (Brahma) 

Normal mesoderm color 

Comblessness (Breda) 

Pea, rose comb 

Colored plumage 

Light shank color 

White skin color 



Sex-linked. 

Almost completely dominant. 
Almost completely dominant. 
Completely dominant. 
Almost completely dominant. 
Almost completely dominant. 
Imperfecth' dominant. 
Almost completely dominant. 
Completely dominant. 
Imperfectl}^ dominant. 
Completely dominant. 
Imperfecth^ dominant. 
Imperfectly dominant. 
Completely dominant. 
Sex-linked. 
Non-sex-linked. 
Imperfectly dominant. 
Imperfectly dominant. 
Sex-linked. 

Imperfectly dominant. 
Imperfectly dominant. 
Completely dominant. 
Completely dominant. 
Imperfectlj' dominant. 
Almost completeh^ dominant. 
Almost completely dominant. 
Imperfectly dominant. 
Imperfectly dominant. 
Completelj' dominant. 
Imperfectly dominant. 
Imperfectly dominant. 
Imperfectly dominant. 
Completely dominant. 
Completeh^ dominant. 
Almost completely dominant. 
Completelj' dominant. 
Completely dominant. 



478 



GENETICS IN RELATION TO AGRICULTURE 



dicate that the character differences depend upon single factor differences. 
As a matter of fact most instances of imperfect dominance are probably- 
due to complex factor interaction, and they, therefore, require further 
study. The list as it stands is more interesting than useful, but it gives 
a rather vivid idea of the variety of character contrasts which may be 
obtained in the barnyard fowl. Aside from a few additions and changes 
in wording, this list is essentially the same as that compiled by Hadley 
for the 1915 "American Poultry Yearbook." 







Fig. 191. — Results of crossing White Plymouth Rock and White Leghorn. A, Pic?, 
White Leghorn; C, Pi 9 , White Plymouth Rock; B,Fi<^, showing a little flecking of black 
and a barred tail feather; D, FiQ , type of barred birds obtained in Fz- (After Hadley.) 



The most important kind of Mendelian work is that which leads to 
some definite analysis of the factor complex characteristic of a given 
breed. As an illustration of such investigations we have Hadley's 
analysis of the genetic constitution of the White Leghorn breed. White 
is often dominant in fowls, so that white breeds may carry latent color 
and pattern factors in their makeup. Hadley finds the White Leghorn 
to be of the genetic constitution CCII{BZ)(BZ), if a male, and CCII 
{BZ)W, if a female, the factors having the following effects; 



MENDELISM IN DOMESTIC ANIMALS 479 

C — a factor for black coloration. The alleloniorpli, c, (Ictcnnincs the 
production of recessive white plumage. 

I — a dominant factor for white pigmentation, which supresses the 
normal production of pigment in the plumage. 

B — the sex-linked dominant factor for barring, a pattern factor, 
which acts on black to produce the familiar j^lumage effect of the Barred 
Plymouth Rock. The recessive, hb, birds are solid color. 

Z — the sex-factor, homozygous in males, and mated to the neutral W 
in females. Hadley found that the White Plymouth Rock was probably 
of the genetic constitution ccn{BZ){BZ); male, or ccii{BZ)W, female; 
and obtained results which accorded with this formulation. From 
matings of White Plymouth Rock females and White Leghorn males, 
Hadley obtained G3 Fi birds, all of which were white, although some of 
them showed a few barred or black-flecked feathers. This is shown in 
the Fi bird in Fig. 191. Mating such Fi birds together should give in F^ 
white and barred birds in the ratio of 13 white :3 barred. The actual 
figures obtained were 134 white :33 barred, a very close agreement with 
expected results. Facts such as these demonstrate with what care the 
breeder must proceed in crossing breeds if he wishes to avoid obtaining 
a heterogeneous mixture of classes in subsequent generations. 



CHAPTER XXX 
ACQUIRED CHARACTERS IN ANIMAL BREEDING 

The problem of the inheritance of acquired characters has been one 
of the historic battlegrounds of biology. Even yet the question is by no 
means settled, although a considerable amount of information has been 
collected about it. Darwin and Spencer both subscribed to the belief 
that acquired characters might be impressed upon the germinal substance 
and therefore of necessity that offspring might inherit such characters, 
for they saw in an intimate relation between soma and germ-plasm a 
powerful method of evolution. It is not necessary longer to question 
the fact of evolution, but the method of evolution still awaits a satis- 
factory solution. 

In animal breeding especially the question of the inheritance of 
acquired characters is of primary importance because much of the func- 
tional activity of animals depends for its perfection upon carefully de- 
veloped training. It is not enough for the race horse to have a good 
inheritance, it is further necessary that it should be developed and trained 
in accordance with methods known to be favorable to the bringing out of 
its inborn qualities, and this is also true in one respect or another of 
other domestic animals. Now it is only natural for those who have 
carefully attended to the development of the inherent characters of their 
livestock to hope and to expect that their efforts have added something 
of excellence to the hereditary complex of the individual. This in brief 
is the interest which the inheritance of acquired characters has for the 
practical animal breeder. 

The Scientific Problem. — Before taking up the evidence as to the 
inheritance of acquired characters, it is necessary to define as clearly 
as possible what is meant by an acquired character, and to determine 
what sort of proof is necessary in order to establish the inheritance of 
such characters. As many writers have pointed out, much futile dis- 
cussion upon the subject has been due to a lack of rigid definition of 
terms. 

Weismann distinguished between blastogenic and somatogenic charac- 
ters. The former were such characters as have their origin in the germ- 
plasm, and the latter are those which are produced by responses of the soma 
or body to surrounding conditions or to its own activities. These latter 
somatogenic variations are the acquired characters of evolutionary litera- 
ture. Shull's definition of acquhed characters is, perhaps, somewhat more 

480 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 481 

precise, namely that acquired characters are modifications of bocUly 
structure or habit which are impressed upon the organism in the course of 
individual life. This distinction is by no means another instance of the 
hair-splitting proclivities of modern science; it is on the contrary a real 
distinction of fundamental importance in shaping conceptions of evolu- 
tion and heredity. It is unnecessary to give any specific examples of 
blastogenic characters, since the whole discussion of Mendelian heredity 
in preceding pages has been confined to them. Of somatogenic charac- 
ters, however, it is perhaps well to mention a few in order to give a con- 
crete starting point for the following discussion. Acquired characters 
include a vast number of characters due to environmental effects, for 
example, small size when a consequence of reduced food supply or other 
conditions unfavorable to growth, increased size consequent upon un- 
usually favorable environmental conditions, mutilation, the effects of 
disease, and other modifications of a like character. Those acquired 
characters which have their origin in response to environmental con- 
ditions have often been distinguished from that other class, the motive 
force in the development of which resides in the organism itself, the 
effects of use and disuse. Conspicuous examples of "achieved" charac- 
ters as distinguished from "thrust" characters are increases in the per- 
fection of function dependent upon exercise, such as the increased speed 
of the trained race horse and the increased sharpness of intellect of the 
trained mind. 

As Thomson has stated it, the precise question at issue is this : Can 
a structural change in the body, induced by some change in use 
or disuse, or by a change in surrounding influence, affect the germ cells 
in such a specific or representative way that the offspring will through its 
inheritance exhibit, even in a slight degree, the modification which the 
parent acquired? 

Obviously a problem such as this must require very critical treat- 
ment, and much, if not all of the evidence brought forward in support 
of the belief in the inheritance of acquired characters suffers from failure 
to fulfil the requirements of a rigid proof. Thomson has given an excel- 
lent extended treatment of this side of the case, as well as of the subject 
of acquired characters in general. 

To satisfy the rigid requirements of an experimental proof any evi- 
dence of the inheritance of acquired characters must fulfil the following 
conditions : 

First, a specific character or modification in the soma must be im- 
pressed upon the organism by a known factor in its environment or in 
its exercise of bodily function. 

Second, the character or modification should be new. There must be 
no question of the reappearance of ancestral traits or characters, or of 

31 



482 GENETICS IN RELATION TO AGRICULTURE 

the specific relation of the determining factor to the character or modifica- 
tion in question. 

Finally the induced change in the organism must reappear in succeed- 
ing generations in the absence of the original factor which determined 
its production. Other conditions in the life of the offspring must remain 
unchanged. The change in question may exhibit a lesser degree in the 
immediate descendants in the absence of the original stimulus, and in 
succeeding generations it may become progressively less, but the critical 
point is the determination of whether such a change is exhibited in any 
degree whatsoever by offspring produced in the absence of the original 
stimulus. 

The Belief in the Inheritance of Acquired Characters. — ^Lamarck 
first stated clearly the belief in the inheritance of acquired characters, 
and the part which it has been supposed to play in the determination 
of the characters of living beings as they exist today. From his observa- 
tions, he formulated two laws, which he stated as follows according to 
Elliot's translation: 

In every animal which has not passed the limit of its development, a more 
frequent and continuous use of any organ gradually strengthens, enlarges and 
develops that organ, and gives it power proportionate to the length of time it has 
been so used; while the permanent disuse of any organ imperceptibly weakens 
and deteriorates it, and progressively diminishes its functional capacity until 
it finally disappears. 

All the acquisitions or losses wrought by nature on individuals, through the 
influence of the environment in which their race has long been placed and hence 
through the influence of the predominant use or permanent disuse of any organ ; 
all these are preserved by reproduction to the new individuals which arise pro- 
vided that the acquired modifications are common to both sexes, or at least to 
the individuals which produce the young. 

It is a curious fact which has been pointed out by Lankester that 
these two laws are mutually contradictory. The first law states that 
adaptive changes occur when organisms are subjected to new environ- 
mental conditions; the second states that such newly acquired characters 
become a part of the heritage of the individual. In other words accord- 
ing to the first law the old established characters of the organism are 
unable to maintain themselves under new conditions; according to the 
second law it is implied that acquired characters having a much less 
extended history possess a permanence and stability not accorded to the 
older established characters. It should be noted, however, that this 
implication was not what Lamarck emphasized. He dwelt rather upon 
the very gradual, "imperceptible" effects of use or disuse, for example, 
in permanently changing characters. 

The belief in acquired characters is still held by some modern biolo- 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 483 

gists, but in a more refined form. Semon, for example, has proposed the 
" Mnemetheorie " as founded upon two premises. First, that although 
the stimulations of the ''sensitive substance of the organism" disappear 
as such, yet after they cease they leave behind in this same sensitive 
substance changes which he has called Engramme. Second, that these 
"Engramme in the sensitive substance" persist not only in the soma, but 
also under favorable circumstances in the germ cells. This form of the 
"memory theory " of heredity might seem to be a convenient hypothesis for 
explaining the assumed inheritance of modifications resulting from the use 
of organs, but it is difficult to imagine how it would favor the assumption 
of inheritance of modifications resulting from disuse of organs or loss of 
parts through mutilation. Obviously the inheritance of mutilations, in 
spite of a few circumstantial cases, cannot be maintained with any 
degree of conviction. The many generations through which circumcision 
has been practised in the Jew and the deforming of women's feet by the 
Chinese are two instances opposed to it. Dehorning of cattle, docking 
the tails of horses and sheep, clipping the ears of dogs, are instances 
which come within agricultural practice and have no permanent effect 
upon the breed. On the whole the neo-Lamarckians have come to 
believe, therefore, in the inheritance of those acquired characters which 
depend upon use or disuse of organs, achieved characters as distinguished 
from thrust characters. A rather crude example of this belief which has 
of late years obtained some notoriety among livestock breeders is Red- 
field's theory of dynamic evolution. According to this statement of the 
belief, the exercise of any organ or function results in a corresponding 
storage of energy in the germ cells, such that the effects are transmitted 
to the next generation. The idea receives practical application from the 
further consequence, that this storage of energy having been granted, 
developed animals must of necessity possess more of it than those un- 
developed, and consequently such animals produce superior offspring. 
E. Davenport, Marshall, Pearl and others have taken issue with Redfield 
upon this subject and have demonstrated clearly that the facts which 
have been cited in support of his theory of dynamic evolution may be 
interpreted with far greater probability in other ways. In fact, the 
biological basis for such assumption as the storage of energy in germ cells 
is very slight. Moreover, the theory is evidently based upon a naive 
disregard of known biological facts, and a non-critical interpretation of 
statistical data. The matter deserves mention here, not because of any 
merit in it, but solely because of the publicity which has been accorded 
it in various journals devoted to practical breeding interests. 

As an example of the kind of agricultural data which those who 
believe in the inheritance of acquired characters point to for support 
of their views, nothing is more striking than the rise and improvement of 



484 



GENETICS IN RELATION TO AGRICULTURE 
Table LXIII. — Reduction of the Trotting Record 



Name of horse 


Place of record 


Date 


Record 


Boston 


Philadelphia, Pa. 
Jamaica, N. Y. 
Philadelphia, Pa. 
Philadelphia, Pa. 
Hoboken, N. J. 
Hoboken, N. J. 
Jamaica, N. Y. 
Jamaica, N. Y. 
Jamaica, N. Y. 
Kalamazoo, Mich. 
Buffalo, N. Y. 
Milwaukee, Wis. 
Boston, Mass. 
Buffalo, N. Y. 
Oakland, Cal. 
Chicago, 111. 
Providence, R. I. 
Cleveland, Ohio. 
Stockton, Cal. 
Terre Haute, Ind. 
Galesburg, 111. 
Terre Haute, Ind. 
Columbus, Ohio 
Readville, Miss. 
Memphis, Tenn. 
Lexington, Ky. 


Aug. 25, 1810 
Oct. 3, 1826 
Nov. 21, 1834 
Oct. 16, 1838 
July 18, 1839 
Oct. 13, 1845 
July 2, 1849 
July 14, 1853 
Sept. 2, 1856 
Oct. 15, 1859 
Aug. 14, 1867 
Sept. 6, 1871 
Sept. 2, 1874 
Aug. 3, 1878 
Oct. 25, 1879 
Sept. 18, 1880 
Aug. 1, 1884 
July 30, 1885 
Oct. 20, 1891 
Sept. 28, 1892 
Sept. 19, 1894 
Sept. 26, 1900 
Aug. 2, 1901 
Aug. 24, 1903 
Oct. 24, 1903 
Oct. 8, 1913 


2:48M 
2:431^ 
2 -37 


Trouble 

Sally Miller 


Edwin Forest 


2 -361^ 


Dutchman 


2 :32 


Lady Suffolk 

Pelham 


2:29J^ 

1 2 • 28 


Highland Maid 


2 :27 


Flora Temple 

Flora Temple 


;2:24K 
2:19% 

2:17M 
2 :17 


Dexter 


Goldsmith Maid 


Goldsmith Maid 

Rarus 


2:14 

2:1314 
2 : 12% 
2:10% 
2 : 10 


St. Julien 

MaudS 


Jay-Eye-See 


Maud S 

Sunol 


2:08% 
2:08% 
2 :04 


Nancy Hanks 


Alix 


2:03% 
2:03% 
2:02% 
2 :00 


The Abbot 


Cresceus • 

Lou Dillon 


Lou Dillon 


1 :58M 


Uhlan 


1 :58 







the American Standard bred horse during the past century. As some indi- 
cation of this improvement we reproduce here Table LXIII, which shows 
how the trotting record has gradually been reduced. Not all of the de- 
crease in the record represents a real advance, for along with improvement 
in potential ability have gone improvements in methods of training and 
in the circumstances under which records have been made. Aside from 
these factors, however, the reduction in the record does indicate very 
strikingly the improvement which has taken place in the American 
trotter. It is a grave question, however, whether any of this improve- 
ment can be ascribed to the inheritance of acquired characters, for such 
a position fails to evaluate the effect of rigid selection which has been 
followed in building up the American Standard bred. This matter 
will be treated further in the next chapter. 

The Argument against the Inheritance of Acquired Characters. — Just 
as Lamarck was the moving spirit in formulating the belief in acquired 
characters, so Weismann was the leading protagonist of the contrary 
opinion. Weismann was forced to this position by his belief in amphi- 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 485 

mixis as a cause of variation, but he found other and abundant evidence 
to confirm his scepticism. Even at the present time his reasons for the 
opposing position are vahd and sound. They may be stated briefly as 
follows : 

1. There is no known mechanism by which the soma may influence 
the germ-plasm in a specific fashion. 

2. The evidence which has been presented in support of the behef 
in the inheritance of acquired characters in not a single case satisfies the 
rigid requirements of an experimental proof. 

3. The theories of the continuity of the germ-plasm and of germinal 
variation can account for all known facts of heredity without resorting 
to the inheritance of acquired characters. 

These statements represent a formidable indictment of the behef 
in the transmission of effects of somatic modification. Some of the 
evidence in support of these statements is given in what follows. 

The Soma and Germ-plasm. — That there is no known mechanism 
by which the soma may influence the germ-plasm in a specific fashion is a 
fact admitted alike by neo-Lamarckians and Weismannians. But, as the 
former point out, an admission of this point by no means necessarily 
includes a denial of the existence of such a mechanism. The present 
knowledge of biochemical relations within the body is in a lamentably 
inadequate condition to serve as a basis for either the denial or affirmation 
of specific relations between body and stirp. 

Fortunately, however, some definite experiments have been performed 
which throw light upon this question. Of experiments on ovarian 
transplantation those of Castle and Phillips deserve the greatest confi- 
dence because they Were performed with animals the genetic behavior 
of which was known. The account of one successful experiment follows: 

On January 6, 1909, the left ovary was removed from an albino guinea-pig, No. 
27, then about 5 months old, and the ovary of a pure black guinea-pig about a month 
old was fastened near the tip of the uterine horn, distant a centimeter or more from 
the site of the ovary removed. One week later, January 13, a second operation was 
performed, in which the right ovary of the albino was removed, and as a graft was in- 
troduced the ovary of a second young black guinea-pig, of like age with the first 
but of different ancestry. After the albino had fully recovered from the second 
operation, she was placed with an albino male. No. 654 with which she remained until 
her death about a year later. 

On the 23rd of July, 198 days after the operation, she gave birth to two female 
young. One was black but bore a few red hairs. . . . The other young one 
was likewise black, but had some red upon it, and its right forefoot was white. 

On October 15 the grafted albino bore a third young one, a male which, like those 
previously borne, had a few red hairs interspersed with black .... 

On January 11, 1910, the grafted albino was observed to be pregnant for the third 
time, and this time she was very large. Unfortunately, on February 2nd, she died of 
pneumonia with three full-grown male young in utero. The skins of these animals were 



486 GENETICS IN RELATION TO AGRICULTURE 

saved. . . . Like the other three young they were black, but with a few red 
hairs among the black ones. They bore no white "hairs .... 

Female 1970, daughter of the grafted albino, was mated with the albino male, her 
father, and bore three young, two of which were albinos and one black with some red 
hairs; If female 1970 had been the daughter of a pure-black mother, instead of a 
grafted albino, we should have expected her to produce an equality of black and of 
albino young. The observed result was the nearest possible agreement with this 
expectation. 

A control mating of the albino male, 654, was made with a female of pure-black 
stock. As a result there were produced two litters of young, including five individuals, 
all black, with red hairs interspersed. This result shows that the red hairs found on 
the six young of the grafted albino was due, not to foster-mother influence of the 
grafted albino, but to influence of the male parent. The young of the grafted mother 
were exactly such in color as the black guinea-pig which furnished the graft herself 
might have been expected to bear had she been mated with male 654 instead of being 
sacrificed to furnish the graft. The white foot borne by one of the young furnished no 
exception to this statement. Spotting characterized the race of guinea-pigs from 
which the father came. He was himself born in a litter which contained spotted 
young whereas neither the pure-bred black race that furnished the graftnor the albino 
race that received it was characterized by spotting. 

Inasmuch as the offspring of albino parents are invariably albirtos, it is certain 
that the six pigmented offspring of the grafted female were all derived from ova 
furnished by the introduced ovarian tissue taken from a black guinea-pig. This tissue 
was introduced while the contained ova were still quite immature, and it persisted 
in its new environment for nearly a year before the eggs were liberated which produced 
the last litter of three j'oung. These young, like the earlier litters, gave no indication 
of foster-mother influence in their coloration. 

The conclusion is forced upon us that the egg-cell during its growth 
does not change in germinal constitution. Its growth is like the growth 
of a parasite or of a wholly independent organism : what it takes up serves 
as food; this is not incorporated merely in the growing organism, it is 
made over into the same kind of living substance as composes the assimilat- 
ing organism. Thus a critical experiment designed to test the relation 
of soma to germ cells with respect to coat coloration failed to demonstrate 
any direct interrelation whatever, and further experiments by the same 
investigators indicated that for other factors also the foster-mother 
exerted no influence whatever on the developing ova. The ovary of the 
black guinea-pig produced exactly the same kind of ova in the body of the 
albino as it would have produced had it remained in the body of the black 
guinea-pig. Figs. 192 and 193 show the nine animals reported in this 
experiment. The full record of this experiment of Castle and Phillips' 
shows in detail the character of critical investigation which has been 
brought to bear on various phases of the question of the inheritance 
of acquired characters. 

In passing, it should be mentioned that a few previous experiments 
on germinal transplantation appeared to indicate the existence of some 
influence of the foster-mother. Of these the experiments of Guthrie on 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 



487 



fowls are alone entitled to any considerable treatment. Outhrie believed 
he obtained evidence of foster-mother influence in a number of cases of 
ovarian transplantation. But, as Castle and Phillips point out in their 
discussion of these cases, the genetic behavior of I lie fowls concerned in the 




Fig. 192. — Guinea-pigs used in experiments on germinal transplantation. 1, a young 
black guinea-pig about 3 weeks old, type of the animals from which ovaries were taken; 2, 
albino female No. 27 into which the ovaries from the black guinea-pigs were transplanted; 
3, the albino male No. 654 which was mated to No. 27. (After Castle and Phillips.) 

experiments was imperfectly known and the results were such as lent 
themselves to more logical interpretation as cases of ovarian regeneration. 
C. B. Davenport in fact repeated these experiments in a more critical 
fashion and found that in every case ovarian regeneration occurred, and 



488 



GENETICS IN RELATION TO AGRICULTURE 



the transplanted tissue failed to function. Accordingly as the matter 
stands at present with regard to the relation of soma and stirp it has not 
been demonstrated that any mechanism for the direct influence of the 




Fig. 193. — The offspring of the pair of albinos shown in Fig. 192. 4 and 5, young 
black females of the first litter; 6, young black male of the second litter; 7, 8, 9, skins of the 
black males found in utero after the death of No. 24. (After Castle and Phillips.) 



germ cells by the body exists; and such critical experimental evidence 
as has been obtained indicates that probably no such mechanism does 
exist. 

The Isolation of the Germ-plasm. — Many biologists see in the con- 
tinuity of the germ-plasm and in its apparent isolation from the soma an 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 489 

insuperable difficulty to an acceptance of the possibility of any germinal 
influence by the soma. It is true that Spencer's theory of the unceasing 
flow of protoplasm through the body, and Darwin's theory of pangenesis 
by which the body cells were supposed to throw off gennnulcs which pass 
to the germ cells were advanced to account for a direct relation between 
body and soma, but even in more refined form such hypotheses have 
received not the slightest verification. The present conception of the 
complex organization of the germinal material, which has been outlined 
in detail in the first part of this text, adds to the difficulties in the way of a 
conception of an interrelation between body and stirp. Weismann em- 
phasizes this difficulty in the statement that the belief that a functional 
modification may be reflected in the corresponding constituents of the 
germ-plasm "is very like supposing that an English telegram to China 
is there received in the Chinese language." This, however, is undoubt- 
edly an overstatement of the difficulties involved, for the nuclear con- 
stituents of each and every body cell is on the whole the same as that of 
the germ cells. Accordingly it is not inconceivable that a bodily effect 
might be impressed upon the germ cells by hormones liberated into the 
blood stream by the nuclear constituents of affected body cells. 

The Inadequacy of Affirmative Evidence. — All the evidence which 
has been presented in support of the inheritance of acquired characters 
fail to satisfy some one or other of the conditions necessary for a rigid 
proof. Attention has already been called to the fact that the transmis- 
sion of the effects of mutilation definitely can be denied. For twenty- 
two generations Weismann cut the tails off mice at birth, yet there was 
no effect upon the length of the tails of new-born mice. 

The case for environmental effects appears to be in no better con- 
dition. Here in particular we meet with the kind of evidence which 
practical men consider favorable to the theory of the inheritance of 
acquired characters. The hardy little Shetland pony has been bred for 
centuries on the rocky islands of Shetland where climate is unfavorable 
and provender often scarce. What more natural than to assume that 
this scantiness of food has had cumulative, stunting effect from gen- 
eration to generation until now the average height is only from 40 to 42 
inches, and many are much smaller? This belief is further strengthened 
by statements to the effect that under more favorable conditions there 
is a progressive increase in size. Thus we find in one account of the 
Shetland pony this statement, "On the prairies of the American corn 
belt the pony tends to increase in size from generation to generation." 
The italics are ours, for the fact must be emphasized that an increase in 
size would not of itself be evidence of inheritance of an acquired 
character, even though it persisted through any number of generations. 
An increase in size under more favorable conditions is indeed to be 



490 GENETICS IN RELATION TO AGRICULTURE 

expected, and it should persist as long as the ponies are kept under 
the more favorable conditions. It would then be merely an instance 
of the reimpressment of a given environmental effect on each succeed- 
ing generation, not a case of the transmission of an acquired character 
at all. But if the modification increases in degree from generation 
to generation, as we may be led to believe by the above statement, 
then we have something very like the inheritance of an acquired 
character. Unfortunately we have no actual concrete evidence on this 
point, and we have no instance of the subjection of the case to the third 
point of proof which has been outlined above, if such increased size 
should persist when the ponies are transported to their original habitat, 
rigidly excluding the possibility of any effects of selection, then the 
evidence of transmission might be accepted. 

This, of course, is not an isolated instance of the supposed trans- 
mission of acquired characters; on the contrary, the agricultural litera- 
ture is full of statements which indicate a tacit acceptance on the part 
of the authors of the inheritance of acquired characters. Large-sized 
breeds come from regions of correspondingly abundant food supply, 
small-sized ones from regions of scanty provender. The small size of 
Alderney cattle has been favored by systematic underfeeding. Sheep 
transported to a dry climate acquire with succeeding generations a more 
and more marked harshness of wool. Instances like these may be 
multiplied indefinitely, but they are all very much alike; they are state- 
ments of opinion rather than of fact, and their interpretation is based 
upon a non-critical treatment of uncertain data. Very often it is ex- 
ceedingly difficult to separate and to evaluate accurately the particular 
effects of different factors in a given instance. Moreover, cases which 
very closely simulate the inheritance of acquired characters may be readily 
imagined. Thus a Shetland pony dam by systematic underfeeding has 
been severely stunted in its growth. Now it is hardly conceivable that 
such a pony could provide for her young while in utero or during the 
suckling period the necessary food for its most favorable development, 
particularly if the systematic underfeeding of the dam continued during 
this period. It is conceivable that such an effect might last through 
several generations when the individuals in question were placed under 
more favorable conditions, and there would be a simple physiological 
reason for the fact. But this would not be in any critical sense, a trans- 
mission of an acquired character, for the germinal material would remain 
the same throughout all these changes. The proof of the transmission 
of acquired characters requires along with it proof that the germinal 
material has been affected in a fashion corresponding to that of the soma. 
A case such as the above would be more properly an effect of propagated 
environment, if we may use such a term, and this might well account 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 491 

for all the increase in size of Shetland ponies from generation to generation 
in the corn belt. To our knowledge it has not been determined accurately 
how great such an effect may be, nor how long it may persist. This is a 
point of some practical interest, but as to its relation to the inheritance 
of acquired characters, it is well to emphasize the fact that, as Thomson 
states, "experiments on increased size of parts are more decisive than 
those which refer only to the size of the whole." 

An experiment designed to supply this need of data on change in 
size of particular body parts was conducted by Sumner, who subjected 
white mice to extreme temperature differences from the time of birth 
until 5 days before the females gave birth to their young. It was 
found that the offspring of warm-room mice, although themselves reared 
under identical temperature conditions with the offspring of cold-room 
mice, presented differences of the same sort as had been brought about in 
their parents through the direct effect of temperature, viz., differences 
in the mean length of tail, foot and ear. Unfortunately the data, as 
Sumner points out, give evidence of considerable heterogeneity in the 
genotypic composition of the population of mice used in the experiment. 
Furthermore no control lots from the same stock of mice were reared 
under average temperature conditions for comparison, and the pregnant 
females were not removed to the common temperature room until 
after the young had been carried in utero for 2 weeks. When, in 
view of these uncertainties, it is learned that in only three out of twenty- 
one cases of statistical comparison of the offspring of warm-room and 
cold-room parents is the actual difference more than 4 times the 
probable error of that difference, it appears that the evidence hardly 
warrants any definite conclusions. The investigation is mentioned here 
in order that the student may realize something of the difficulties in- 
volved in attacking this general problem. 

The Transmission of Functional Modifications. — There finally re- 
mains the question of the transmission of the effects of use and disuse, 
and this in a sense is the field in which most tenacious adherence to the 
doctrine of the inheritance of acquired characters is found. Use and 
disuse was one of the chief factors considered by Lamarck in his 
attempt to account for change in species; use and disuse with Darwin, 
in spite of his open hostility to Lamarckism, was an important factor 
in the evolution of species. Use and disuse was supposed to account for 
the blindness of cave fauna, for the reduction in size of wings of the ostrich 
and emu, for the loss of legs by snakes, and for a host of other similar 
structural changes. 

In animal breeding it is in this category perhaps that the inheritance 
of acquired characters assumes its greatest practical importance. The 
development of speed in race horses has already been referred to. But 



492 GENETICS IN RELATION TO AGRICULTURE 

although it might seem at first glance a very simple matter to account 
for the increased speed of the American trotter in successive generations 
by use inheritance, nevertheless in the light of modern conceptions of 
germinal constitution, the simplicity of this explanation is a snare and a 
delusion. Similarly the inheritance of milk- and butter-fat producing 
capacity in cattle and goats, the inheritance of the propensity to lay on 
flesh in meat-producing animals, and other characters of domestic animals 
of great practical importance simulate in the way they have been built 
up, it cannot be denied, the inheritance of acquired characters. But 
simulation is far from proof, and any attempt to examine the records 
from the standpoint of rigid proof cannot fail to impress the student with 
the unsatisfactory nature of the material involved. Practical breeding 
operations have been designed to produce results, not to prove or dis- 
prove any particular theory of inheritance. Consequently every vari- 
able which favors the production of results is made use of, so that it is 
only very rarely that a single variable occurs in a given set of practical 
data. As the number of variables increases it becomes more and more 
difficult to assess to each its particular value. To illustrate the diffi- 
culties of interpreting data such as we obtain from practical breeding 
operations, we need merely call attention to some of the important vari- 
ables which enter into such results, such as original germinal diversity, 
mutational changes, effects of selection, effects of functional modification, 
increasing knowledge of methods of developing animals, and maintenance 
of more favorable environmental conditions. The effect of all of these 
variable factors often enters into the end result in practical breeding 
operations. It is possible to determine statistically by means of rigid 
experimental analysis just how much is due to each one of them, but 
unfortunately this has not been done. For the present then we must 
conclude that it is a non-critical, unscientific attitude of mind which 
would assign to one of these variable factors, viz., the effects of func- 
tional modification, a leading importance in the end result, particularly 
when it is the most debatable one of all. Certainly we are in need of 
rigidly controlled experiments along this particular line. 

Parallel Induction. — It is a well-known fact that the germ cells are 
susceptible to injury under unfavorable conditions such as occur at times 
in the body. Accordingly under adverse conditions there is a possibility 
that the germ cells may be affected along with the body. The first 
experimental evidence definitely establishing this fact was obtained by 
Fischer, who subjected pupse of the moth, Ardia caja, to a low tem- 
perature and thereby produced a distinct new form with much darker 
wings, the males being darker in color than the females. By mating a 
pair of these 173 offspring were reared of which 17 resembled their parents 
in being much darker colored than the species type and again the males 



ACQUIRED CHARACTERS IN ANIMAL BREEDING 493 

were darker than the females. There is a real distinction between this 
and similar cases of parallel induction, where the stimulus acts directly 
upon the germ cells, and the supposed action of a stimulus through a 
somatic modification on the germ-plasm. So far as practical breeding 
operations go this matter perhaps has little importance save in relation 
to disease and immunity, under which head it will be discussed later. 
The problem comes up for the most part in connection with the effect 
of adverse conditions upon the individual. Thus conceivably alcoholism 
in many cases may result in such a thorough poisoning of the entire system 
that body and stirp are both injured. In consequence offspring of such 
parents might display structural peculiarities and defects, similar to or 
different from those produced in the parent by the same adverse condi- 
tions. Certain experiments on the effects of alcohol on the progeny of 
animals furnish the direct evidence of parallel induction. Stockard's 
investigation with guinea-pigs led to the conclusion that "mammals 
treated with injurious substances such as alcohol, ether, lead, etc., 
suffer from the treatments by having the tissues of their bodies injured. 
When the reproductive glands and germ cells become injured in this 
way they give rise to offspring showing weak and degenerate conditions 
of a general nature and every cell of these offspring having been derived 
from the injured egg or sperm cell is necessarily similarly injured and can 
only give rise to other injured cells and thus the next generation of off- 
spring is equally weak and injured, and so on. . . This might be 
construed to show the transmission of acquired characters, but it 
cannot be properly interpreted in such a sense. There is in this case no 
transmission of a new or strange character strictly speaking, merely a 
weakened or injured cell gives rise to other weak cells." On the other 
hand Pearl, working with chickens, reaches the conclusions, (1) "that 
the progeny of alcohohzed parentage while fewer in numbers is made 
up of individuals superior in physiological vigor, and (2) that this result 
is due to a selective action of the alcohol upon the germ cells." Nice, 
also, who worked with white mice, fails to observe any injurious effect 
from alcohol in fertility or vigor of growth and but a small one in via- 
bility. Thus the evidence now available would certainly indicate that 
it is dangerous to draw far-reaching conclusions as to the general effects 
of poisons on the germ cells from data obtained on a single species. 

Modern breeds of livestock without question trace back to extremely 
diverse foundation stocks. This historical fact has been discussed very 
inadequately in Chapter XXVII, and it has been shown specifically in 
some cases that the potentialities for high performance existed early in 
the breeds. Accordingly the constant practice of breeding from the best 
has resulted in the elimination from the line of descent of a large propor- 
tion of those animals which have failed to measure up to the standard 



494 GENETICS IN RELATION TO AGRICULTURE 

of performance. Selection has indeed been a powerful factor in the 
improvement of modern breeds of livestock, but it is not necessary to 
assume that selective improvement is due to anything other than the 
isolation and multiplication of animals possessing the best combinations 
of germinal elements. Mutations or germinal changes sometimes occur. 
They are not necessarily favorable, but when useful they are likely to 
be preserved. They add to the store of heritage in our breeds of livestock. 
Favorable conditions of this kind may be maintained, and to the non- 
critical mind produce results simulating the inheritance of acquired 
characters. 

The Conclusion. — There appears to be no escape from an attitude 
of extreme scepticism with respect even to the transmission of functional 
modifications. There is no conclusive experimental demonstration in 
the true sense of the word of the inheritance of any acquired character, 
and there is abundant experimental evidence, although it cannot of 
course be conclusive, for the contrary belief. Accordingly it should be 
evident to the practical breeder that anything which is so difficult for 
scientists to demonstrate must have little possibility of practical value. 
There is enough experimental evidence to demonstrate that it cannot 
possibly be the rule for the great majority of characters, and that it 
cannot produce significant effects in short times. The individual breeder 
works with relatively few generations, and he should shape his operations 
in accordance with that fact. 

But although the inheritance of acquired characters may be denied, 
particularly from a practical standpoint, that denial does not carry with 
it any under-estimation of the importance of modifiability in animal 
breeding operations. Thomson has given this point happy expression 
in the statement that, " Although what is ' acquired ' may not be inherited, 
what is not inherited may be acquired." And also just as some of 
the data of practical breeding operations seem to indicate an inheritance 
of acquired characters, so some methods of breeding the success of which 
is apparently based upon the inheritance of acquired characters really 
depend for their success upon harmony with other laws of heredity. 
A change in theoretical interpretation need not necessarily change 
breeding methods. 



CHAPTER XXXI 
THE SELECTION PROBLEM IN ANIMAL BREEDING 

There are two general views respecting the effectiveness of selection, 
namely, that it depends upon the isolation of hereditary material of the 
most excellent kind, or in metaphor that selection separates the gold 
from the dross, and secondly that it is due to modification of germinal 
elements, that by selecting in a specific direction the hereditary material 
itself is actually molded in a corresponding fashion. The two inter- 
pretations are fundamentally at variance each with the other; one 
interpretation would have it that the hereditary elements are relatively 
constant, at least that they are not subject to gradual continuous change; 
the other interpretation favors a belief in factor inconstanc}^ insignificant 
fluctuating variability as it were in factors themselves, which provides 
opportunity by appropriate selection for actual modification of germinal 
elements. These opposed views have already been discussed at some 
length in other chapters, it remains to apply the conclusions therein 
reached to practical animal breeding operations; and to discuss certain 
other phases of the selection problem which are of particular interest 
in practical work. 

In spite of vigorous statements to the contrary, there seems to be 
little reason to doubt that the isolation view of selection can account for 
all results which have been communicated thus far with respect to this 
question, whether of practical experience or of experimental research. 
In the preceding chapter we had occasion to consider, in passing, the 
reduction which had occurred in trotting records during the past one 
hundred years. Data of this kmd are not lacking in the annals of agri- 
culture, and are often offered as. evidence in support of the belief in the 
gradual accumulation of favorable increments in the development of 
breeds. But although this old Darwinian idea of improvement in animals 
is very generally subscribed to, at least verbally, b}' practical men, it 
is nevertheless true that whenever a critical examination is made of 
specific instances the support for it largely disappears. 

The American Standard Bred Horse. — In Table LXIII are given the 
data which indicate how the trotting record has been lowered during 
the histor}'- of racing in America. A parallel table might be given to 
show how the pacing record has been reduced. Now although these 
records have been used on the one hand in support of belief in the inheri- 

495 



496 GENETICS IN RELATION TO AGRICULTURE 

tance of acquired characters and on the other hand in advocacy of the 
doctrine of the creative effect of selection, there appears to be Httle 
foundation for either of these views when the evidence is analyzed 
critically. 

The moving spirit in the establishment and improvement of the 
American Standard bred has been the demand for fast race horses. 
Throughout the history of the breed performance has been emphasized 
strictly and consistently; the judgment of merit has been based upon 
actual track records, or upon the ability to produce animals of superior 
performing ability. The requirements for registration adopted in 1882 
by the National Association of Trotting Horse Breeders are an eloquent 
testimonial of this. They are given in full below. 

In order to define what constitutes a trotting-bred horse, and to establish a breed 
of trotters on a more intelligent basis, the following rules are adopted to control 
admission to the record of pedigrees. When the animal meets with the requirements 
of admission and is duly registered, it shall be accepted as a standard trotting-bred 
animal. 

First. — Any stallion that has, himself, a record of 2:30 or better; provided any of 
his get has a record of 2: 40 or better; or provided his sire or his dam, his grandsire or 
his grandam, is already a standard animal. 

Second. — Any mare or gelding that has a record of 2:30 or better. 

Third. — Any horse that is the sire of two animals with a record of 2: 30 or better. 

Fourth. — -Any horse that is the sire of one animal with a record of 2:30 or better; 
provided he has either of the following additional qualifications: (1) a record himself 
of 2:40 or better; (2) is the sire of two other animals with a record of 2:40 or better; 
(3) has a sire or dam, grandsire or grandam, that is already a standard animal. 

Fifth. — Any mare that has produced an animal with a record of 2:30 or better. 

Sixth. — -The progeny of a standard horse when out of a standard mare. 

Seventh. — The progeny of a standard horse when out of a mare by a standard 
horse. 

Eighth. — The progeny of a standard horse when out of a mare whose dam is a 
standard mare. 

Ninth. — Any mare that has a record of 2 :40 or better, and whose sire or dam, 
grandsire or grandam, is a standard animal. 

Tenth. — A record to wagon of 2 : 35 or better shall be regarded as equal to a 2 : 30 
record. 

These are rules well calculated to sort out and preserve the most 
excellent lines of descent. That they have operated by discovery and 
utilization of unusually excellent genotypes is rather strikingly indicated 
by a study of some of the famous families of standard-bred animals. In 
Table LXI V E. Davenport has presented data relative to the ten greatest 
producers of speed in the history of American racing up to and including 
1901. Without exception these ten sires belong to the famous Hamble- 
tonian family of racing horses. They are all descendants of Hamble- 
tonian 10: for Belmont 64 is a grandson of Hambletonian 10, and Geo. 
Wilkes and Happy Medium were both sons of Hambletonian 10. Within 



THE SELECTION PROBLEM IN ANIMAL BREEDING 



497 



Table LXIV. — The Tkn Greatest Producers op Speed up to and Including 1901 

{After E. Davenport) 



Sires 


Sired by 


Trotters 


Pacers 


Total 


Nutwood 600 


Belmont 64 
Hambletonian 10 
Geo. Wilkes 519 
Geo. Wilkes 519 
Geo. Wilkes 519 
Happy Medium 400 
Geo. Wilkes 519 
Geo. Wilkes 519 
Geo. Wilkes 519 
Geo. Wilkes 519 


131 

158 

124 

116 

102 

94 

82 

89 

49 

78 


34 
2 
34 
41 
47 
20 
23 
14 
52 
21 


165 


Electioneer 125 


160 


Onward 1411 


158 


Red Wilkes 1749 


157 


Alcantara 729 


149 


Pilot Medium 1579 


114 


Simmons 2744 


105 


Wilton 5982 

Gambetta Wilkes 4651 

Baron Wilkes 4758 


103 

101 

99 







a few generations, therefore, this famous family of racing horses has 
produced a remarkable series of performers, horses which have been 
able to trot or pace a mile within 2 : 30. There seems to be little question, 
therefore, that this family of fast horses had its foundation in the care- 
ful fostering of the favorable genotypic material of Hambletonian 10; 
and a transmission of it through a relatively small number of exceptional 
sires which may have possessed a genotypic arrangement somewhat 
superior to that of Hambletonian 10, as the record of Geo. Wilkes 519 
in particular might indicate. Davenport has made a very careful study 
of the records in the Register and Yearbook, a study which should be 
continued and extended. Without considering in any detail the ex- 
tensive data which have been collected, it appears fairly certain that 
selection in the improvement of trotting and pacing horses has operated 
by detecting and multiplying the most favorable genotypes; and that 
training, in so far as it has had influence, has served as a means of developing 
inborn potentialities to the full, and, therefore, of detecting most favorable 
lines of descent. 

Fecundity in Fowls. — Pearl's investigations on the inheritance 
of fecundity in fowls have already been touched upon, but they deserve 
more extended treatment at this point, for in them the relative effective- 
ness of phenotypic and genotypic selection is strikingly contrasted. 
For if performance has anything to do with development of more favorable 
hereditary material, or if selection has a creative effect in a given direc- 
tion, then it would appear to be a conclusion unavoidable that mass 
selection must result in increased average winter egg production. Yet 
as a matter of fact, as shown graphically in Fig. 185 there was actually 
a slight decrease in average winter egg production during a 9-year period 
of such selection. 

That this selection was rigid and a fair demonstration of the ineffect- 

32 



498 GENETICS IN RELATION TO AGRICULTURE 

iveness of purely phenotypic selection for fecundity in the Barred 
Plymouth Rock is indicated by the plan which was followed during 
this portion of the investigations. Only pullets were used for breeding 
stock which had laid 150 or more eggs during their first laying year, 
and cockerels were selected from among the progeny of 200 egg hens. 
The type of selection practised was, however, strictly mass selection, for 
the selected birds were bred together without respect to genetic relation- 
ship, and no tests were made of the laying capacities of progenies from 
particular matings. This last point is of particular importance, because 
it definitely distinguishes the method of breeding used as one typically 
of mass selection. 

Obviously the reason for the ineffectiveness of selection during this 
period of mass selection lies in the fact that modifiability in fecundity 
is very great. This particular fact has been discussed fully in a pre- 
ceding chapter, but here it must be considered again as the reason for 
the fact that this system of selection failed to result in improvement in 
egg-laying capacity, for were performance and genotypic constitution 
closely correlated, then this system of mass selection should have been 
effective. But as a matter of fact the criterion of selection used in this 
portion of the investigations, namely total yearly egg production, was 
evidently not a good index of genotypic constitution, for apparently 
it failed to distiguish between individuals belonging to a number of 
intergrading genotypes. Consequently, whenever, by chance a female 
was selected which by phenotypic variation represented the upper 
limits of her genotypic class, the population was thereby thrown back 
by that much to the level representing the mean phenotypic performance 
of her particular genotypic class. A wide range of modifiability for each 
genotype, therefore, continually held the average yearly production 
down to the original value for the population. 

But beginning with the year 1908 a radical change was made in the 
method of selection. During the first portion of the second period, 
the object was merely to ascertain the actual mode of inheritance of 
fecundity, a subject which is discussed more fully elsewhere; but during 
the second portion, from 1912 to the present time, selection was only 
carried out for high egg production. Essentially, however, the mode of 
selection during these two portions of the second period was the same so 
that we may consider this as a single period. The performance index 
during this period was winter egg production rather than total egg 
production. But in the selection of high winter producers for breeding 
purposes, a progeny performance test was employed as well as an actual 
individual performance test. Every female which was selected during 
this period came from a high-producing mother, the female progeny of 
which were all high producers. In case such a female failed to give 



THE SELECTION PROBLEM IN ANIMAL BREEDING 



499 



progeny of high performance in the first year she was not retained for 
further breeding purposes. Males for breeding purposes were selected 
on a like rigid basis; thoy were from high-producing mothers, the daugh- 
ters of which were all high producers, and any male was rejected imme- 
diately if his progeny failed to measure up to high standards. Complete 
individual pedigrees were kept during this period. For the sake of 
comparison low and mediocre strains were also selected on a basis equally 
rigid for their particular characters. 

The success of this type of selection is strikingly evidenced by the 
data set forth in Table LXV, which gives the means from which Fig. 



Table LXV. — Mean Winter Egg Production of the Maine Station Barked 
Plymouth Rock Flocks from 1899-1915 {Data of Pearl) 



Laying year 


Mean winter 

production of 

all birds 


Number of 
birds making 
winter records 


Mean winter pro- 
duction of all 
birds selected for 
high production 


Mean winter pro- 
duction of all 
birds selected for 
low production 


1899-1900 


41.03 
37.88 
45.23 
26.01 
26.55 
35.04 
40.65 
22.44 
19.93 
26.69 
31.76 
30.49 


70 
85 
48 
147 
254 
515 
635 
653 
780 
359 
247 
264 
232 
182 
192 
179 


54.16 
45.57 
50.58 
57.42 
52.61 
52.20 
45.89 




1900-1901 




1901-1902 




1902-1903 




1903-1904 




1904-1905 




1905-1906 




1906-1907 




1907-1908 




1908-1909 


22.06 


1909-1910 


25.06 


1910-1911 


17.00 


1911-1912 


35.93 


16.43 


1912-1913 


43.01 
52.20 
45.89 




1913-1914 




1914-1915 








Total and means 


35.05 


4,842 


51.49 


20.14 



185 was constructed. From an interpretative standpoint, therefore, the 
direct contrast is brought out sufficiently well in this case, for when 
selection was placed on a fairly rigid genotypic basis it was immediately 
successful. It seems hardly possible to explain the facts of this series of 
investigations by any other than an appeal to the isolation view of 
selection, particularly when consideration is directed toward the rapidity 
with which genotypic selection established high-producing strains in a 
flock which had failed to respond to a rigid system of mass selection. 

Bantam Fowls. — In a previous chapter evidence was presented tending 
toward the general conclusion that the most potent source of that varia- 



500 GENETICS IN RELATION TO AGRICULTURE 

tion which has been made use of in the estabUshment of modern breeds 
of domestic animals has come from amphimixis, the result of polyphyletic 
origin. But modern breeds have become highly standardized and they 
are closely guarded by rigid rules of pedigree registration. In conse- 
quence improvement within them has been effected purely by methods 
of selection without hybridization between breeds. In poultry, however, 
and here again we turn to Pearl for data, new breeds are still being 
created, and almost entirely bj'- methods of hybridization. Thus for 
practically every variety of larger domestic fowl there is a corresponding 
bantam variety. Pearl sent queries to bantam breeders in all parts of the 
world for the purpose of obtaining information upon the method of 
creating new varieties of bantams. Here it would seem was an ideal test 
for the utility of Darwinian selection in the establishment of breeds, for 
it would appear to be a very direct mode of attack in breeding bantams 
corresponding to a given larger variety of fowl simply to select for smaller 
size within the larger breed. Pearl's enquiries brought out the fact, 
however, that in no case were bantam breeds created in this fashion, but 
always by crossing the larger breed with some bantam varietj^ and then 
selecting within the hybrid progeny of subsequent generations. In view 
of the demonstration of the relatively simple Mendelian heredity of body 
weight in Seabright X Hamburg crosses which has been demonstrated 
by Punnett and Bailey, we can readily understand how this method 
should be quickly and uniformly successful. The creation of bantam 
varieties of fowls, therefore, strikingly bears out this general thesis, that 
so far as the results of selection go in actual practice, the isolation 
interpretation is sufficient to account for all facts. The creative view 
of selection is an uncertain doctrine to accept for guidance. 

Selection and Breeding Methods.^ — Finally it becomes necessary to 
again point out that a change in interpretation does not necessarily call 
for a change in breeding methods. In fact it can be shown that in general 
those breeding methods have been most successful which are most closely 
in accord with the isolation view of heredity. This fact is particularly 
patent when the earlier histories of breeds is taken into account, and the 
vast amount of inbreeding which has been employed by the best of these 
old pioneer breeders is given a true valuation. For inbreeding determines 
the fixing of a given genotypic constitution, because in such a system of 
breeding family excellence is the basis of judgment, just as in the suc- 
cessful selection for high winter egg production progeny test was the 
basis of selection for breeding stock. That most famous of all early 
breeders, Robert Bakewell, when he set about establishing his herds 
made excursions all over Britain for the purpose of selecting and purchas- 
ing the best of all sorts for his foundation stock. However, after he had 
brought this stock together he used only the progeny of these animals in 



THE SELECTION PROBLEM IN ANIMAL BREEDING 



501 



his herd, no other animals were introduced from outside sources. Of 
necessity, therefore, his was a method of close inbreeding, and he did 
not shrink for a moment from using this method to the fullest extent. 
The same method of breeding characterizes the work of other early 
breeders. Thus one of the most famous of early Shorthorn bulls. Comet 
(155) was very closely inbred as shown by the pedigree in Fig. 194. 
At public auction this great bull was sold in 1810 for 1000 guineas, 
a very high price at that time. He was considered the crowning achieve- 



Favorite (252) 



Bolingbroke (86) 



Foljambe (263) 
[ Young Strawberry 



Foljambe (263) 
[ Phoenix \ 

I Favorite 



Comet (155) { 



Favorite (252) 



Bolingbroke (86) 



[ Foljambe (263) 



Phoenix 



Young 
Phoenix 



I 



[ Young Strawberry 

(Foljambe 
Favorite 



Phoenix 



f Foljambe (263) 
[ Young Strawberry 



Fig. 194. — The pedigree of Comet (155), an illustration of extreme inbreeding in Shorthorn 

foundation stock. 



ment of Charles Colling's notable career as a breeder of Shorthorn cattle. 
The extremely close breeding shown in this sample pedigree from a 
notable herd of Shorthorn cattle may again be used as an argument in 
support of the isolation interpretation of selection in successful practical 
breeding operations. 

In fact throughout the entire history of animal breeding, improve- 
ment has been most strikingly referable to the influence of a limited 
number of families and individuals of superior excellence, a fact which 
speaks strongly for the isolation view of selection. Every breed has its 
famous animals and families, and every breeder who has studied pedigrees 
at all must have been impressed by the small percentage of early animals 
which are represented in almost every pedigree of present-day individuals 
of the breed. The "search for the prepotent sire" and full utilization 
of him when discovered have been the central features of the breeding 



502 GENETICS IN RELATION TO AGRICULTURE 

methods of many a successful animal breeder. But prepotency, if it 
indicates anything, points to the supreme importance of genotypic con- 
stitution as the measure of excellence, and not to augmented excellence 
from development, performance, or any other factor, for these prepotent 
animals are often not themselves superior in individual excellence to 
many other animals of the breed although the latter have failed to 
impress the breed so strongly with their characteristics. 

It is safe to say, therefore, that the isolation view of selection is 
sufficient to account for any of the results which have been obtained in 
practical breeding operations. Since practical breeding methods have 
often been very successful it follows as a matter of of course that the 
adoption of such an interpretation does not involve of necessity any altera- 
tion in livestock-breeding methods. Why then emphasize the importance 
of this interpretation? The answer to the question should be clear. It 
follows that, if a method of breeding is not creative with respect to 
addition of new and better elements to the hereditary material, full 
utilization must be made of those existing elements of the germinal 
material which are of most value commercially. It, therefore, follows 
that no method of breeding, however excellent, can attain a full measure 
of success unless the very best existing foundation stock is utilized, for 
in such individuals alone are contained in the very best combinations 
those hereditary elements upon the utilization of which the breeder must 
rely for success. The importance of extreme care in the selection of 
foundation stock cannot, therefore, be overestimated. 

Selection Indices. — In all selection work, as indeed in all practical 
breeding, it is necessarj^ for the stockman to have his ideal thoroughly in 
mind. In the absence of such a definite ideal, it is difficult to understand 
how any breeding operations can possibly have an orthogenetic trend. 
The requirements of efficient selection, therefore, demand first an ac- 
curate method of judging the comparative worth of a series of animals with 
respect to certain definite characters, and second a method of weighting 
different characters in the same individual according to their comparative 
value from a breeding standpoint. It is here that science may be ex- 
pected to give definitely useful contributions to practical breeding 
methods in the determination of mathematically accurate means of 
comparing data. 

With respect to the first point, the comparative value of a given 
character in a series of individuals, several factors must be considered. 
Only one definite case can be considered here, that of milk production 
in dairy cows. Obviously with cows kept under identical conditions, 
two factors have a great influence on the amount of milk produced within 
a given period, namely, the age of the cow and the stage in lactation. 
Pearl has shown that the amount of milk produced by a cow within a 



THE SELECTION PROBLEM IN ANIMAL BREEDING 



503 



given period is a logarithmic function of her age. His investigations 
further show that the curve of milk production with respect to age is 
of the general form 

Y = a + hX + cX^ + d\og X. 

In this equation, Y denotes the amount of milk produced in a given 
time, X the age of the cow, and a, h, c, and d arc constants the value of 
which must be calculated for different breeds. Stated in general terms, 
therefore, milk production increases with age until at about 5 years 
a maximum is reached, after which it decreases with advancing age. 
With reference to milk production within a given lactation period, it is 
of course a matter of common experience that milk production decreases 
gradually during the lactation period. These two factors have been 
taken into account by Pearl in preparing a table of comparative effi- 
ciency percentages for dairy cows. In this table the maximum efficiency 
is set at 100 per cent, and the comparative excellence at any given age 
or stage of lactation is given a value in per cent, of this maximum value. 
Pearl has calculated such a table for 24 months of which 10 months 
only are given in Table LXVI. This table provides a very satis- 
factory method of comparing a set of individual records from cows of 
different ages and in different stages of lactation. 



Table LXVI. — Efficiency Percentages for Milk Production in Dairy Cattle 

{Data of Pearl) 



Age of cow in years and months 






Months since 


freshe 


ning 




























' 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 




Per 


Per 


Per 


Per 


Per 


Per 


Per 


Per 


Per 


Per 




cent. 


cent. 


cent. 


cent. 


cent. 


cent. 


cent. 


cent. 


cent. 


cent. 


1:6 to 1:11 


58 


54 


51 


47 


44 


41 


37 


34 


30 


27 


2:0 to 2:5 


73 


69 


64 


60 


56 


52 


48 


43 


39 


35 


2:6 to 2:11 


82 


77 


72 


67 


62 


57 


52 


47 


41 


36 


3:0 to 3:5 


89 


83 


77 


71 


66 


60 


54 


48 


43 


37 


3:6 to 3: 11 


93 


87 


81 


75 


69 


62 


56 


50 


44 


38 


4:0 ,to 4:11 


97 


91 


84 


78 


71 


65 


58 


52 


45 


39 


5:0 to 5:11 


100 


93 


86 


79 


72 


66 


59 


53 


46 


39 


6:0 to 6: 11 


100 


93 


86 


79 


72 


65 


58 


52 


45 


38 


7:0 to 7:11 


99 


92 


85 


78 


71 


64 


57 


51 


44 


37 


8: 00 to 8: 11 


97 


90 


84 


77 


70 


63 


56 


50 


43 


37 


9: 00 to 9: 11 


94 


88 


82 


75 


69 


62 


55 


49 


43 


36 


10 :00to 10: 11 


91 


85 


79 


73 


67 


61 


54 


48 


42 


36 


11 :00to 11 : 11 


88 


82 


76 


71 


65 


59 


53 


47 


41 


35 


12 : 00 to 12 : 11 


85 


80 


74 


68 


63 


57 


52 


46 


40 


35 


13 :00to 13 : 11 


82 


76 


71 


66 


61 


55 


50 


45 


39 


34 



504 



GENETICS IN RELATION TO AGRICULTURE 



The second difficulty has to do with determining some method by 
which selection may be made effective for a number of characters, or 
in other words a method of comparing different individuals with respect 
to a number of different characters. The use of selection index numbers 
provides a way of surmounting this difficulty. Pearl has made use of 
the following fundamental selection index in the Maine Station poultry 
work: 

5(a + b) 
^^ c + d + 1 
in which formula: 

Ii = selection index number for a particular individual. 

a = percentage of eggs which hatched. 

b = percentage of eggs actually laid to the total number which might 
have been laid during the breeding season, February 1 to June 1. 

c = percentage of infertile eggs. 

d = percentage of chicks which died within 3 weeks after hatching. 

The application of this index to different hens in a breeding flock is 
shown in Table LXVII. The selection index ranges in value from 2.8 to 

Table LXVII. — Selection Indices for Barred Plymouth Rock Pullets (Data of 

Pearl and Surface) 



Band 
No. 


a 


6 


. 


d 


Zi 


Band 

No. 


a 


b 


c 


d 


Ii 


10 


21 


18.33 


36.0 


33.33 


2.8 


29 


28 


28.33 


27.0 





10.0 


160 


9 


15.83 


42.0 





2.9 


23 


39 


35.83 


28.0 


8.33 


10.0 


402 


14 


33.33 


30.0 


50.00 


2.9 


428 


46 


40.08 


20.0 


22.22 


10.0 


352 


12 


41.67 


14.0 


60.00 


3.6 


122 


49 


34.17 


15.0 


23.53 


10.5 


358 


50 


31.67 


32.0 


69.23 


4.0 


375 


41 


37.50 


36.0 





10.6 


438 


35 


35.00 


45.0 


37.50 


4.2 


712 


42. 


50.00 


20.0 


20.00 


11.2 


441 


38 


20.00 


33.0 


33.33 


4.3 


408 


48 


45.86 


16.0 


22.73 


11.8 


21 


25 


35.00 


14.0 


44.44 


5.0 


38 


61 


37.50 


9.0 


28.00 


13.0 


393 


12 


47.50 


9.0 


50.00 


5.0 


731 


27 


35.83 


23.0 





13.1 


705 


38 


26.67 


34.0 


25.00 


5.4 


395 


29 


37.50 


24.0 





13.3 


717 


24 


21.67 


19.0 


20.00 


5.7 


443 


56 


54.17 


6.0 


35.29 


13.3 


39 


23 


29.17 


26.0 


16.67 


6.0 


409 


37 


46.67 


4.0 


25.00 


13.9 


377 


32 


37.50 


16.0 


33.33 


6.9 


771 


43 


60.00 


13.0 


22.22 


14.2 


746 


59 


28.33 


15.0 


47.06 


6.9 


19 


68 


24.17 


24.0 


6.66 


14.5 


87 


36 


39.17 


17.0 


35.71 


7.0 


152 


26 


39.17 


11.0 


9.09 


15.5 


359 


61 


32.50 


15.0 


50.00 


7.1 


366 


52 


25.83 


13.0 


7.14 


18.4 


442 


41 


68.33 


38.0 


28.57 


8.2 


768 


74 


45.00 


20.0 


9.38 


19.6 


400 


44 


47.50 


16.0 


38.10 


8.2 


434 


72 


58.33 


17.0 


14.28 


20.2 


27 


18 


40.00 


8.3 


25.00 


8.5 


750 


57 


52.50 


16.0 


10.00 


20.3 


757 


23 


51.67 


10.0 


30.77 


8.9 


770 


71 


50.83 


9.8 


12.82 


25.8 


725 


33 


29.17 


6.0 


27.27 


9.1 


752 


48 


59.17 


6.0 


12.50 


27.5 


112 


17 


46.67 


18.0 


25.00 


9.3 


397 


38 


40.00 


6.0 


5.89 


30.3 


753 


61 


47.50 


46.0 


10.53 


9.4 


168 


88 


35.83 


4.7 


13.89 


31.6 


407 


41 


41.67 


18.0 


23.53 


9.7 


749 


57 


46.50 


4.0 


6.45 


45.2 



THE SELECTION PROBLEM IN ANIMAL BREEDING 



505 



45.2, those birds having the highest breeding value which have the highest 
selection index. By this method it is possible to substitute for a vague 
personal impression of breeding value, an exact numerical expression 
which is an accurate measure of the breeding value of any individual. 
It is possible to devise such selection index numbers for other purposes, 
and they should prove of utility in practical breeding operations. 

Another line in which still further necessity for strictly scientific 
analysis is exemplified is that of detailed study of curves of production. 
Thus Pearl and Surface have made a detailed biometrical study of the 
seasonal distribution of egg production in domestic fowls. From this 
study they find that the polygon of monthly egg production is of the form 
shown in Fig. 195. They find that with pullets the normal season of egg 



IC 








































\ 














14 

§ 
■•§12 

3 
£,0 
















N 


k 














/ 


^ 


/ 








\ 


^ 








Pj 10 




/ 


/ 
















\ 






^6 




/ 




















\ 






/ 




















s 


> 


4 

9 





























Nov, Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. 

Fig. 195. — Diagram showing the weighted mean monthly egg production for each month 
of the pullet year. {After Pearl and Surface.) 

production begins in November. The mean rises rapidly during the 
following 2 months, but in February there is a characteristic slackening 
up in egg production. In March and April egg production is at a maxi- 
mum, and after that it decreases fairly regularly until it reaches a mini- 
mum in October, with the exception of a slight, but significant, indenta- 
tion in May. These data taken together with certain other facts which 
have been determined during the course of the Maine Station investi- 
gations of egg production indicate that the laying year may be broken 
up into four periods which correspond broadly with natural cycles of 
egg production in the domestic fowl. The first of these periods begins in 
November and ends at about March 1. The end of this winter-laying 
period is marked rather definitely in the curve of annual egg production 
by the distinct slackening of increase in egg production during February. 
The winter period of laying is in a sense an added period for it is not nor- 



506 GENETICS IN RELATION TO AGRICULTURE 

mal to the wild fowl, and some fowls do not show this winter cycle at all. 
The period of March, April and May on the other hand is the natural 
reproductive cycle of the fowl. As might be expected it is the period 
of maximum egg production. It is terminated by the onset of the 
brooding period, which is indicated in the figure by the shght indentation 
during May. The third period, June, July and August, is characterized 
by a falling off in mean monthly egg production. This of course is the 
period during which chicks are reared, and it represents also a prolonga- 
tion of the normal spring cycle. Finally, following the summer cycle, 
there is a period of 2 months, September and October, during which 
molting normally occurs. It is a period of minimal egg production. 
Pearl and Surface have made a careful and detailed investigation of each 
of these periods of laying, only the main results of which are indicated 
in the above account. However, one practical result of the investigation 
which has been employed extensively by them in subsequent investiga- 
tions is the determination of the nature of the winter cycle of egg pro- 
duction, and its value as an index of egg laying capacity. This is a cycle 
added to those normally found in wild fowls, it is fairly sharply marked 
off from other cycles, and it represents an invariable characteristic of 
high laying individuals. It is not participated in by all individuals, and 
as we shall show later it has been subjected to definite Mendelian formu- 
lation. Accordingly there is abundant justification for using winter 
egg production as an index of egg producing capacity rather than using 
the longer total yearly production as such an index. The work of record 
taking in practical breeding operations having in mind the production of 
strains of superior laying ability has been simplified and made more accu- 
rate by these investigations. It is thus plainly to be seen how very im- 
portant it is that as much detailed knowledge as possible be collected 
about any character with which the breeder is working. 

Correlation and Selection. — Many of the "points to be observed in 
judging" or "aids to selection" of domestic animals are based upon 
empirical knowledge of the correlation which exists between conforma- 
tion and performance. These have been mentioned in earlier chapters 
and are presented in various texts and manuals on judging, selecting 
and breeding, and need not be discussed here. 

The practical value of a statistical knowledge of correlation between 
somatic characters and functional variations was mentioned in Chapter 
III; also the importance of biological soundness in the material used in 
investigating such correlations. Very few investigations have been 
conducted in this field of scientific research, but the increasing utilization 
of statistical methods is an earnest of future progress in this direction. 
A fine illustration of what the biometrician can do in this line is found 
in the recent work on the correlation between body pigmentation and 



THE SELECTION PROBLEM IN ANIMAL BREEDING 507 

egg production in the domestic fowl by Harris, Blakeslcc antl Warner. 
This study dealt with the relationship between the concentration of 
yellow pigment in the ear lobe of White Leghorn hens and their egg re- 
cords of the preceding months. It was found that there is a very close 
interdependence between October ear-lobe color and the egg production 
of the pullet year. 

"Expressed in absolute instead of relative terms, the correlations 
determined indicate that on an average birds differing by 5 per cent, in 
the amount of yellow in the ear lobe will differ by about 7 eggs in their 
annual production. Thus the difference is one of real practical signific- 
ance. For example, birds showing only 10-20 per cent, of yellow in their 
ear lobes in October will have laid on an average about 185 eggs each, 
whereas birds exhibiting 55-65 per cent, of yellow will have an average 
annual production of only about 130 eggs." 

These results prove that in the fowls used in this investigation color 
of the ear lobe (and presumably, of the leg, beak and vent) would have 
served as a practical index for selection of high layers. The authors 
point out, however, that the flocks from which their data were obtained 
represent a selected class of birds (in certain egg laying contests). "Such 
birds show, because of better breeding, better feeding and care, or both, 
a far higher annual egg production than the average flock. Unfortun- 
ately data of the kind presented here are not as yet available for the 
unselected class of layers." This correlation has long been known and 
utilized by poultry breeders (according to Kent it was noted in published 
form in 1879) and Blakeslee and Warner made earlier statistical studies 
than the one we have considered. Both Kent and Warner point out that 
degree of pigmentation is only one of several characters that may be 
utilized in selecting high laying hens, the other more important ones being 
time and duration of molting and size of abdomen. 



CHAPTER XXXII 
HYBRIDIZATION IN ANIMAL BREEDING 

Within the past century the tendency in practical breeding operations 
has been toward the upbuilding of different breeds by a process of rigid 
selection. Naturally this method of breeding has looked upon any 
suggestion for the introduction of foreign blood with intolerance, an 
intolerance which is shown particularly in the rigid requirements of 
registration laid down by all breeders' associations. But the work in 
plant breeding in particular has demonstrated that occasionally hybrids 
have a commercial value in themselves aside from their usefulness as a 
source for the establishment of new varieties. Accordingly within the 
past decade there has been a growing tendency to investigate more 
closely the question of hybridization in animals, both in its scientific and 
practical aspects. 

Grading. — In practical animal breeding grading refers to the method of 
improving a herd of animals of indifferent blood by the use of pure-bred 
sires. In the United States this practice has been very common on western 
ranches where the common scrub stock of the range has been graded up 
largely by the use of Hereford bulls, and bulls of other beef breeds. The 
practice is very common in horse breeding throughout the entire nation, 
for a large proportion of the stallions which stand for public service are 
pure-bred animals. In horse-breeding, however, it is regretably too often 
true that the pure-bred sires used in successive generations are not of the 
same breed, consequently the term grading is to be applied to this sort 
of breeding with some reservations. Grading is a practice universally 
commended. Aside from providing a market for a large number of 
pure-bred sires which would otherwise be sent to the shambles, a desid- 
eratum which may account for some of the warm advocacy it has 
received from breeders of pure-bred livestock, it does actually lead to 
notable improvement when practised intelligently, for pure-bred livestock 
which has been selected for many generations for particular utilitarian 
purposes is on the whole very much superior to scrub stock. Moreover, 
since the proportion of pure-bred animals is very low, it is a positive fact 
that any considerable improvement must depend upon some method of 
raising the general level of the great number of inferior animals. 

Perhaps in no line of livestock production are results so readily com- 
parable as in dairy cattle. Here performance is becoming more and 

508 



HYBRIDIZATION IN ANIMAL BREEDING 



509 




Fig. 196.— The results of grading scrub dairy cattle with pure-bred Holstein-Freisian bulls. 
{After Kildee and McCandlish, Iowa A. E. S.) 



510 



GENETICS IN RELATION TO AGRICULTURE 



more the decisive standard of excellence, a fact which is reflected in the 
establishment of advanced registry records by pure-bred dairy cattle 
associations for those animals which prove of superior performing or 
breeding ability. Many experiment stations have enthusiastically 
recommended the use of pure-bred sires in building up dairy herds, 
a few have conducted investigations for the purpose of determining 
precisely how much improvement may be expected from the use of 
such sires. An investigation of this kind at the Iowa Station has pro- 
ceeded far enough to warrant a preliminary report. For this investiga- 
tion seven cows, six heifers, and one young bull were purchased in an 
isolated region of Arkansas where it was practically certain that no 
pure-bred bulls had ever been used. These were developed at the 
station, and their records are available for comparison with those of 
their daughters which have been sired by Guernsey, Holstein-Friesian, 
or Jersey bulls. In Fig. 196 are shown the results of grading two gen- 
erations to pure bred Hol- 
Tablb LXVIII. — Comparison of a Scrub Cow, stein-Friesian bulls. The 
No. 52, WITH Her Daughter, No. 69, Sired first generation of grading 
BY A Holstein-Freisian Bull (Dato of j^as given a heifer which is 

Kildee and McCandhsh) . , ,- . . , , 

intermediate in most char- 
acters between its dam and 
the general type of Hol- 
stein-Friesian cow. She, 
however, lacks the char- 
acteristic color markings of 
the Holstein-Friesian, a 
fact which obscures some- 
what her resemblance to 
that breed's type. The 
second generation calf, her 
daughter, however, pos- 
sesses characteristic Hol- 
stein-Friesian markings and would pass for a very fair specimen of the 
breed. The important consideration, however, is the comparative 
excellence of these animals in milk and butter fat production. The 
data relative to this question are given in Table LXVIII. The most 
notable feature in these records is the increase of 64 per cent, in average 
net returns. 

Table LXIX has been so compiled from the data of Kildee and Mc- 
Candlish as to give a general resume of their investigations. The cows 
have been divided into three lots according to the kind of sire usecl in 
grading. In the first lot scrub cows are compared with their daughters 
which were sired by a Holstein-Friesian bull; in the second lot the scrubs 



No. of 
cow 


No. of 
lactation 
periods 




Pounds 
of milk 


Pounds 
of fat 


Net 
returns 


52 


6 


Ave. 
Best 


3,856.4 

4,588.4 


174.53 
201 . 67 


$19.29 
16.27 


69 


4 


Ave. 
Best 


5,757.4 
6,822.8 


242.31 
283.75 


31.57 
38.65 


Per cent, in- 
crease of daugh- 
ter over dam 


Ave. 
Best 


49.3 

48.7 


38.8 
40.7 


63.7 
? 



HYBRIDIZATION IN ANIMAL BREEDING 



511 



Table LXIX. — Comparative Records of Scrub Dairy Cows and Their Grade 
Daughters (Data of Kildee and McCandlish) 



Lot 



No. 

of 

cows 



No. of 
lacta- 
tion 
periods 



Average yearly records 



Pounds 

of 

milk 



Pounds 

of butter 

fat 



Net 
returns 



Per cent, increased; 
grades over scrubs 



Pounds 
of 
milk 



Pounds 

of butter 

fat 



Net 
returns 



Scrubs 

Scrubs X Holstein . 

Scrubs 

Scrubs X Guernsey 

Scrubs 

Scrubs X Jersej'- . . . 



4 


17 


3,156.6 


158.10 


4 


12 


5,428.5 


230.10 


4 


19 


4,056.1 


181.85 


4 


8 


4,146.7 


194.92 


1 


7 


3,437.5 


166.74 


1 


2 


3,643 . 1 


199.64 



$18.42 
23.86 

24.59 
19.98 

24 . 39 
21.01 



71.90 



2.21 



6,00 



41.50 



7.18 



20.00 



29.9 



-18.7 



13.8 



which produced heifer calves to Guernsey bulls are compared with their 
daughters, and in the third lot a Jersey grade cow is compared with her 
scrub mother. 

This table must be interpreted with some care, because it suffers from 
the same fault which mars a large proportion of agricultural data; things 
are compared which are not directly comparable. In Table LXIX 
have been presented the indices which would have made it possible to 
evaluate these data with respect to milk production and butter-fat content, 
but the evaluation has not been made in this case. The comparison of 
scrub cows with their grade Holstein-Friesian daughters shows substantial 
increases in milk production, butter-fat production, and net returns, and 
this in spite of the fact that immature cows are compared with mature 
ones. The increases here are due to several things. The scrub cows of 
this lot were the most inferior of all, the Holstein-Friesian bull which 
was used probably came from better producing lines within his breed than 
did the bulls of the other breeds, and a more representative test has been 
made of the grade Holsteins than of either of the other two lots of grades. 
If proper allowance had been made for the immaturity of the Holstein 
grades, they would compare even more favorably with their dams. In 
the Guernsey class the average excellence seems to be somewhat lower 
than that of the dams. The scrub cows of this lot, however, were better 
producers than those of either of the other two lots. Moreover, the 
immaturity of the grades in this test has had more effect than in the 
Holstein class, because two of them are represented by first year records 
only. Three of them were sired by a Guernsey bull which had been 
loaned to the station, and he apparently was not a good sire of dairy 
quality. The fourth member of the lot was sired by a Guernsey bull 
belonging to the station and she showed an increase of over 100 per cent., 



512 GENETICS IN RELATION TO AGRICULTURE 

in all points of comparison with her dam. She was easily the most ex- 
ceptional individual reported upon in the investigations; so that there is 
evidently no lack of excellence in the Guernsey breed for grading up 
dairy herds. Finally in the third lot, which consists of two individuals, 
a scrub cow and her grade Jersey daughter, when the immaturitj'' of the 
latter is taken into account, a substantial improvement is displayed. 
The numbers in these investigations so far are not large, but they do 
show that enough improvement results from the use of good pure-bred 
sires to warrant fully the increased expenditure necessary to obtain them. 

In grading, however, as in all other forms of animal breeding, it is 
necessary to observe all the precautions which have been found necessary 
for effecting permanent improvement. These may be stated in the fol- 
lowing general fashion. 

The Sires Used in Successive Generations Must Belong to the Same 
Breed. — It may be to the best financial interests of the stockman some- 
times to breed to a sire of a different breed from the one used in grading, 
but, if that should be done, the cross-bred stock thus produced should not 
be retained for breeding purposes. There are several reasons for insist- 
ing upon strict adherence to such a plan of operations, and they have 
to do mainly with uniformity of the finished product, a requirement 
which can only be met by following a definite, consistent line of procedure. 
If such be followed, the result is to make the herd in the fourth generation 
practically pure bred so far as purposes of utility are concerned. In 
fact, such grade herds, freed as they are from the dictates of fashion 
which often prevail within breeds, are often superior in actual produc- 
tive capacity to pure-bred herds. 

The Sires should be Selected with Strict Regard for the Improvement 
which It is Desired to Effect within the Herd. — It is not enough to use 
any^individual of the breed upon which the choice has been set, for 
paradoxical as it may seem there are some pure-bred mongrels. Within 
most breeds there are families which are notable for certain definite charac- 
teristics. In the interests of uniformity, therefore, the series of sires 
which is selected should ordinarily belong to the same family : they should 
at least conform to a single, specific type. Furthermore, the sire should 
be a superior specimen of his breed. The man who desires to improve a 
grade herd can afford to neglect fancy points entirely, but conformance 
to high utility standards should be insisted upon. Among dairy cattle 
advanced registration records based upon performance are excellent 
indices for judging the possible value of individuals in grading. In other 
breeds of livestock an approximation can and should be made to standards 
of excellence based upon performance. 

Dams should he Selected Strictly According to a Definite Utili- 
tarian Standard. — The rate and extent of improvement in a grade herd 



HYBRIDIZATION IN ANIMAL BREEDING 513 

must necessarily depend to a considerable extent upon the care used in the 
selection of the female breeding stock. A standard of excellence should 
he established for them as well as for the sires, and any which fall below 
that standard when subjected to a fair trial should be promptly eliminated 
from the breeding herd. The standard should be so high that only the 
very best females in the herd will be retained, it should consequently 
be raised gradually as the excellence of the herd increases. The breeder 
should guard jealously against disposing of his best female stock. 

When all this has been done the question naturally arises, through 
how many successive generations is it necessary or advisable to use 
pure-bred sires? To this question only a general answer can be given. 
Improvement in grading is at first rapid and in successive generations 
becomes less and less rapid as the grade stock approaches more and more 
closely to the standard of excellence of pure-bred stock. Practically, it 
appears to be true that four or five generations of grading, particularly 
if rigid selection of both sires and dams be practised, is sufficient to bring 
the standard of utilitarian excellence up to that of the pure-bred animal. 
After that it is a very grave question whether further employment of 
pure-bred sires is either necessary or desirable, provided it is possible to 
select a sire from a large number of high-grade animals. To effect 
improvement in utilitarian excellence in such herds requires sires of very 
superior excellence, and, if pure-bred, they would usually be too expensive 
for use in grade herds. On the other hand, it is possible among a large 
number of grade animals to select a sire from a superior line of grade 
stock, which would perhaps not be surpassed in utilitarian excellence by 
any pure-bred animal. Such grade animals do not have an inflated value 
on account of their. breeding. They come within the price standards of 
sires which may be used in improving grade stock. It is also an open ques- 
tion whether more actual improvement may not be effected in high-grade 
stock of superior excellence by selection within it, rather than by further 
top-crossing to pure-bred sires. There is some probability that such stock 
may be more variable and consequently possess greater potentialities 
for improvement than the pure-bred stock itself. It is a matter of regret, 
however, that grading has not progressed far enough in very many cases 
to make it necessary to consider this problem. As a general rule, there- 
fore, it is best, if possible, to continue breeding to the best pure-bred sires 
which are available. 

In the early days of breed improvement provision was made for admit- 
tance to record of animals which were the result of top-crossing several 
times to pure-bred sires, and the question often arises whether this should 
be resumed, inasmuch as present day livestock associations with only 
unimportant exceptions make no provision for entry of any animal that 
is not the offspring of recorded animals. Fad and fashion play a large 

33 



514 GENETICS IN RELATION TO AGRICULTURE 

part in determining this position, but it must be admitted that it has 
some other, more secure basis than this, namely in the fact that such 
grade animals, particularly when crossed together, more often throw 
animals off-type with reference to breed standards than do pure-bred 
animals. Such off-type animals may not be at all undesirable from a 
utilitarian standpoint, they may simply fail to meet fancy points which 
breed standards insist upon. It is an open question whether livestock 
associations may not find it conducive to advancement to provide some 
method for the infusion of new blood, particularly in breeds which are 
giving increased attention to performance standards. 

Crossbreeding. — Crossbreeding is the term applied to crossing of 
distinct types or breeds for special purposes. For all practical purposes 
the Blue Andalusian fowl is one of the simplest cases of crossbreeding, 
for it represents a simple heterozygous condition, the result of crossing 
Black and Splashed White Andalusians. 

Crossbreeding, although often severely condemned by livestock 
breeders, is by no means a new practice ; and the persistency with which it 
has been followed is in itself some indication of merit. The avowed 
object of crossbreeding is to combine the excellent qualities of both breeds 
or types which are used. Whether that object may be accomplished can 
only be determined by trial, but in general it may be stated that for 
complex functional characters such as speed in horses, milk or beef pro- 
duction in cattle, wool production in sheep; in short for practically all 
utilitarian characters a blended condition is to be expected in the cross- 
bred offspring. The degree of excellence with which crossbred stock 
meets the purposes for which it is bred should be the justification of the 
practice, for sentimental considerations should have little weight in 
dictating practical methods. 

Perhaps the best known kind of crossbred stock is the blue-gray type 
of cattle. These cattle are crosses either between Aberdeen-Angus and 
Shorthorn cattle or more often between Galloway and Shorthorn cattle. 
In the early part of the 19th century the production of blue-gray 
calves by mating the black cows of Scotland with white Shorthorn bulls 
was so common as to arouse grave concern for the future of the Aberdeen- 
Angus breed. The crossbreds were particularly noted for vigor and 
rapid growth, along with high quality, uniformity, and superior utiliza- 
tion of food. The high repute in which these cattle were held was 
apparently based upon superior market excellence, a superiority which 
has been confirmed by more recent trials at the Iowa station. By cross- 
ing two beef breeds, therefore, it is apparently possible to secure an 
animal superior to either one of them from the feeder's standpoint. 
There is here exhibited, therefore, a rather mild form of that hybrid vigor 
which is so often exhibited in crosses between different races within a species. 



HYBRIDIZATIOX T\ AMMAL HRKEDIXG fjlS 

In certain cases, however, crossbreeding has been used for the a\'o\ved 
purpose of employing a given breed for a doubk^ purpose. An example 
of this is the jiractice in some herds of grade Holsteiu-Freisian cattle of 
using A])erdeen-Angus bulls in order to obtain calves which may be 
fattened for tlu; l)aby-beef market. In this case the Holstein-Freisian 
calves are themselves not unsuited to the purpose, and the Aberdeen- 
Angus cross simply gives them increased excellence in quality and early 
maturity. The use of the Dorset ram on Merino ewes for the pioduct ion 
of grade ewes for hot-house lamb production is another instance of cross- 
breeding for a definite pui-pose. There is room for purposeful cross- 
breeding such as this, but for unsystematic crossbreeding without definite 
purpose, condemnation cannot be too severe. 

The reasons for the condemnation of crossbreeding as a systematic 
breeding program are not far to seek. The threatened extinction of 
the Aberdeen-Angus breed in Scotland in the earlj^ 19th century is 
only one phase of the problem. The first and primary reason for such 
disfavor, however, is the neglect of pure-bred stock which follows such a 
practice. The success of crossbreeding depends largely upon the excel- 
lence of the breeding stock which is utilized, but it is probably true that 
minor defects in the foundation stock are often totally obliterated by in- 
creased vigor and excellence in the cross-bred progeny. The temptation 
to lower the high standards of excellence in the pure-bred stock which is 
being used in crossing and to retain all animals which give any promise 
whatever of producing good cross-bred offspring is, therefore, very strong. 
Moreover, for continuation of the practice it is necessary to maintain 
two lines of breeding, one to supply the pure-bred foundation stock for 
crossing and the other to supply the cross-bred animals themselves. 
When only a small part of the herd is set aside for continuing the pure- 
Ijred lines, the number of individuals from which selection may be made 
is so much smaller that the chances of producing superior individuals is 
considerably less. It is also extremely difficult to enforce the rule that 
the cross-bred stock must not be used for breeding purposes. The 
tendency to breed from particularly excellent individuals which are 
sometimes obtained by crossbreeding is very great; but, if yielded to it 
will surely result in loss of the uniformity of type and excellence which 
(characterized the original cross-bred animals, a natural consequence of 
the operation of the Mendelian law of segregation and recombination. 
These are facts the full gravity of which must be realized before embark- 
ing on crossbreeding operations. 

Species Hybridization Among Domestic Animals. — Species hybrids 
among domestic animals are by no means unconnnon, although in large 
part they have been regarded as curiosities rather than as foundation 
sources of breed improvement, or as themselves of practical value. 



516 



GENETICS IN RELATION TO AGRICULTURE 



In this account species hybrids will be treated according to whether they 
themselves are of some value or are possible sources of breed foundation 
stock. 

The hybrids which are themselves of some practical value, so far as 
utilization in this country goes, are confined to the genus Equus and its 
allied genera, and the typical example of this class of hybrids is the mule. 
The mule has proved a very satisfactory draft animal, and at the present 
time it is largely used, particularly in warmer climates, for that purpose 
(Fig. 197). 




Fig. 197. — A choice draft mule. Height 18-2 hands, weight 1900 pounds. An unusually 
heavy mule of excellent type. (After Obrecht.) 

The mule is a hybrid between the mare and the jack; the reciprocal 
cross of the jennet and stallion is called a hinny (Fig. 198). The cross is 
an instance of strict species hybridization and both sexes of the hybrid 
are sterile. The importance of the mule breeding industry in the United 
States may be judged from the fact that according to statistics of 1915 the 
estimated number of mules was about 4,500,000 with a farm value exceed- 
ing $500,000,000. " The average value of mules per head according to U. S. 
Department of Agriculture estimates was $112.36, while that of horses 
was placed at $103.33. These latter figures may be taken roughly as an 
indication of the relative esteem in which horses and mules are held for 
draft purposes, which is practically the only purpose for which mules are 
employed. 

Although the mule breeding industry is of such great magnitude, 
the relative merits of the horse and mule even for draft purposes are still 
in dispute. It is not the purpose of this account to enter into the debate, 



HYBRIDIZATION IN ANIMAL BREEDING 



517 



nevertheless a brief mention of a few considerations which tend to cloud 
the issue cannot be held out of place. The mule at its best, when com- 
pared with the beauty of form of a well-bred horse, suffers greatly. It 
partakes too much of the characters of the ass, ancient symbol of all 
that is silly and u^ly, to excite greatly the admiration of those who have 
sentimental regard for the horse, the close companion of man in battle, 
foray, and chase. A second consideration is the fact, also true in cross- 
l)reeding, that the breeding of mules withdraws permanently from the 
racial stream much of the very best of horse blood. Whil(> inferior mares 




Fig. 198.— a hinny, obtained by mating a jennet to a stallion. {After Mumford.) 

may produce mule colts that are better for draft purposes than any horse 
colts they might produce, nevertheless for the production of the best class 
of mules, it is absolutely necessary to select with care the very best type 
of brood mares. For these reasons, and others of less importance, 
strong partisans of the horse are prone to permit their prejudice against 
the mule and their high regard for the horse to influence strongly their 
judgment of the point at issue, namely this, whether a given lot of 
mares when bred to a good jack will produce mules which are better 
suited for draft purposes than would be the horse colts produced by these 
mares when bred to an equally good stallion. 

By common consent the mule is considered more vigorous, hardier, 
and freer from disease than either parent. Part of these qualities may 
be ascribed to the ass's influence, but certainly these are characters 
common to a large number of species hybrids. The reciprocal cross, 
jennet X stallion, the hinny, is commonly reported to be different from 
the mule. According to Darwin, the male is prepotent in both crosses 



518 



GENETICS IN RELATION TO AGRICULTURE 



so that the hinny resembles the horse more than does the mule. There 
appears, however, to be some question about the explanation of this case, 
and it is a significant fact that Goldschmidt, who mentions common 
report in connection with this case, has seen fit to question the accuracy 
of it. Apparently many of the differences are due to individual differ- 
ences in the animals which have been used, and are, therefore, of no sig- 
nificance for determining differences in reciprocal crosses. 

The fertility of the mule is an everlasting question of dispute, for 
from time to time reports are made of fertile mare mules. Unquestion- 
ably such cases are very rare, and in most cases some doubt may be 
thrown either upon the question as to whether the mare "mule" was a 




Fig. 199.- 



-The marc mule on the left. On the right, her foal by a jack. 
blance in markings of the leg. {After von Wahl.) 



\o\.Q the reseni- 



mule at all, or whether, if a mule, she was not suckling the colt of some 
other dam, for there are abundant authentic instances of mare mules 
which have given milk. Among instances of fertile mules are those 
reported by von Wahl, who discovered two cases in Brazil of mules which 
had produced foals when bred to an ass. The foals were somewhat 
larger than their dams, and were throughout mule-like in appearance. 
Von Wahl reports, also, a case of a foal from a mare mule out of a stallion, 
but did not himself examine it. It is only fair to state that in these 
cases the chain of evidence is not complete. Lloyd-Jones has given a 
resume of the evidence with respect to fertile mules, and has noted some 
new cases, all of which are, however, questionable. Mares apparently 
occasionally exhibit mule-like characters, and many of the cases reported 
appear to depend upon a mistake of such a mare for a mule. Figs. 199, 200 
and 201 seem to represent an instance of this kind. 



HYBRIDIZATION IN ANIMAL BREEDING 



olO 



The male mule apparently never produces functional spermatozoa. 
Wodsedalek has found that the horse and ass have different numbers of 
chromosomes, so that the mule comes from the union of an ep;g-cell con- 




FiG. 200.— A mare mule (?) with a foal by a Perdieron stallion. {Afkr Llou'l-Jon',.) 




racters in its feet 



Fig. 201.— The lual ui Fit;. 200. It is sai.l t.. .■xl.il.n mul.-lik.- .Iku 

and actions, although otherwise its characters are all hurse-hke. {AJtcr Lloyd-Joncs.) 

taining nineteen chromosomes with sperm cells containing either thirty- 
two or thirty-three. In consequence reduction divisions in the mule 
are prevailingly abnormal as to chromatin distribution, and no functional 
spermatozoa appear to be produced. If we judge by analogy with plants, 



520 



GENETICS IN RELATION TO AGRICULTURE 



however, there appears to be no reason for questioning the possibility of 
the occasional production of functional sperm and egg-cells, although un- 
questionably they would be very rare. In that case it is difficult to see 
why mare mules when bred to stallions may not occasionally produce 
foals which would either be very horse-like or very mule-like in appear- 
ance. On this basis, however, we are justified in regarding with grave 
doubt those cases of mare mules which have produced several foals, for 
cases of fertility should be isolated phenomena which should rarely be 
repeated in the same animal. The matter of fertile mules possesses 
some theoretical interest, but little practical importance. 




Fig. 202. ^A zebroid, produced by mating a burro to the Grevy zebra. (After Roimnel.) 

Hybrids between other species of Equus have been obtained from time 
to time, and some attempts have been made, as yet unsuccessful, to find 
a place for them in practical agriculture. The zebra crosses with the 
horse and the ass, producing in both cases vigorous, growthy hybrids. 
In Fig. 202 is shown a zebra-ass hybrid, in Fig. 222, a Grevy zebra, and 
in Fig. 224, a zebra-horse hybrid. Other hybrids have also been pro- 
duced but they are all simply zoological curiosities. They all appear to 
be infertile, like the mule, although here again reports are not agreed. 
Darwin mentions a zebra-ass hybrid which when bred to a mare got a 
foal, very horse-like in appearance. Rommel reports the zebra-ass 
hybrid to be infertile, although giving greater promise of a limited 
fertility than the mule. 

In the genus Bos a number of species hybrids are known. Detlefsen 
has given a list of them taken from Nathusius. Thus the domestic 
cow. Bos taurus, has been mated with the yak, Bibos grunniens; with the 



HYBRIDIZATION IN ANIMAL BREEDING 521 

gayal, Bibos frontalis; with the gaur, Bibos gaurus; with the zebu, Bos 
indicus, and with the American bison, Bison americanus. Apparently 
all these hybrids are fertile in the female sex, and sterile in the male. 
They are commonly more vigorous than the parents, and in some cases 
they provide a source for breed improvement for specific purposes. 
These features are discussed in the next following chapter, in which 
illustrations also will be found. 

Many other species hybrids have been reported among domestic 
animals. Thus there are persistent reports of hybrids between the sheep 
and goat. In fowls a great variety of strange hybrids have been pro- 
duced. Brentana lists thirteen remarkable hybrids in the Phasianidae 
among them peacock-guinea fowl, pheasant-fowl, guinea fowl-fowl, 
and various pheasant species crosses. Although of technical interest these 
hybrids do not promise to yield anything of commercial value. 



CHAPTER XXXIII 

DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 

In the present chapter not only the subject of disease, but the related 
subjects of defects and immunity will be dealt with from the genetic 
standpoint. According to the strict definition of the term, disease is an 
abnormal process exhibited in some part of the body and dependent for 
its initial impetus upon an external cause. Obviously if a definition such 
as this be applied, there could be no such thing as the inheritance of disease, 
but genetic research has demonstrated bej^ond the shadow of a doubt 
that conditions in the body which more or less directly predispose it to 
disease are inherited. The subject, therefore, deserves treatment adequate 
to the need of outlining clearly the relation of problems of disease to 
inheritance. 

The Inheritance of Disease. — The problem of the inheritance of disease 
is one very much like the problem of the inheritance of acquired characters, 
for it is hedged around with confusion of every day thought and the same 
type of misconception that characterizes this latter problem. Thomson 
has recognized these elements of difficulty and has given the subject, 
particularly as it relates to human inheritance, an adequate, extended 
treatment. 

Many of the misunderstandings which have arisen have been derived 
not only from common folk lore, but from loose thinking on the part of 
those who practice the medical profession as well. It is not always an 
easy matter to distinguish between inheritance of disease and inheritance 
of predisposition to disease, although the distinction is one readily 
conceivable from a theoretical standpoint, and necessary for clarity of 
thought. It is necessary, moreover, to emphasize the fact that reappear- 
ance of disease in successive generations does not constitute inheritance. 
It is particularly true in the human race that successive generations are 
often subjected to the same conditions of life. If these be unfavorable, 
any predisposition to disease of a specific kind may result in the reappear- 
ance of the disease. Illustrations of this sort occur in every day observa- 
tion ; they include such things as the tendency to tuberculosis in certain 
families, the persistent reappearance of gout in successive generations, 
nervous disorders which are expressed in various forms in a given line of 
descent. Finally it is not common in every day thought to draw dis- 
tinctions as finely as is necessary in scientific treatment. It is possible, 

522 



DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDINd 523 

for instance, for the fetus to be infected with disease at any time from the 
moment of conception to that of birth, and ante-natal infections are as 
truly acquired as are post-natal infections. It is possible for the fetus 
to be infected directly and independently of the mother, as occurs at 
times in cases of venereal disease; it is also possible for the fetus to become 
infected from the mother through the placenta, a much less common 
phenomenon. In either case, however, it is confusion of thought to speak 
of such instances as examples of the inheritance of disease. The many 
ramifications of the subject cannot be dealt with fully here; suffice it to 
say that as with acquired characters so with disease transmission the 
affirmative case has not been proven. 

The Inheritance of Predisposition to Disease.^Predisposition to 
disease may be either specific or general. In specific predisposition 
the individual is liable, should prejudicial conditions occur, to contract 
a particular disease, as for example gout, whereas general predisposition 
gives room for infection with a series of similar diseases. This latter 
type of predisposition is very commonly expressed in general consti- 
tutional debility rather than in any specific fashion. It is particularly 
shown in nervous diseases in man, where a predisposition to nervous 
derangement is expressed in a variety of ways in a given line of descent, 
l:) ut eve n here the predisposition is often specific to a remarkable degree. 

[That predispositions are inherited cannot well be questioned, but in 
tracing their transmission it is necessary to guard against factors which 
confuse cases. Thus it does not necessarilj'^ follow that an individual 
jjredisposed to a disease should invariabl^^ become a victim to that disease. 
Transportation in early life to a new environment or particular attention 
throughout life to matters making for health with respect to a known 
inborn tendency may entirely overcome the predisposition. The degree of 
predisposition also varies; in some cases it is so strong as to amount 
practically to transmission of disease; in other cases it is relativelj^ 
weak, and requires either that the inciting cause be impressed frequently 
upon the individual before resistance is broken down and the individual 
yields to attack or that some other violently unfavorable condition 
should enter into the life of the individual. 

It-49-wDTth*^whiie-reHwirking tharfpVedisposition to disease like many 
defects of structure often skips generations in inheritance. In general it 
appears that the normal state of health is the dominant condition and 
that predispositions are for the most part dependent upon the action of 
recessive factors or sets of factors. Accordingly predisposition to disease 
is more likely to crop out as a result of consanguineous matings, because 
related individuals are more likely to bear the same recessive factors in 
their germinal material. In this fashion it is not difficult to account for 
the persistent atavistic appearance of disease in some lines of descent. 



524 



GENETICS IN RELATION TO AGRICULTURE 



The Inheritance of Defects. — By defects we refer specifically to 
abnormal structures or processes which are the almost invariable ex- 
pressions of particular genetic constitutions. Of these a vast number are 
known, particularly in man where in recent years a considerable amount 
of attention has been devoted to them. A very familiar example is 
haemophilia, a bodily condition such that the blood does not possess the 
ability to clot when blood-vessels are ruptured. The defect in man is 
determined by a sex-linked recessive factor and exhibits the same type of 



1.^ 2, 



II 






1 J_ 2 J_ si 4I 5 1 gI T sI 9_L lul 1] I 12, 

y ■ ■ • 4 #|earh-12N O ■ • • C 



4 #f earli-i2N 
f early 



l'i"i'6"6 



Fig. 203. — Pedigree of a family of "bleeders" — the K. family, located in and about Car- 
roll Co., Maryland. The son, II, 2, was a bleeder but died without issue. The eldest son, 
III, 1, of the daughter, II, 3, was a bleeder from 18 up to 45 years, "often bled till he 
fainted." He had 2 unaffected brothers and .3 normal sisters but 1 sister, III, 10, was a 
"bleeder until 40." He had a son, IV, 1, who was a very bad bleeder from 18 until toward 
middle life and a daughter, IV, 2, who often " bled until she fainted " and eventually died of 
dysentery. All 19 children of the 2 normal brothers were normal and 9 children of the 
normal sister. III, 7. The affected sister, III, 10, had 3 sons and 2 daughters who were 
affected. IV, 5, is stated to be "a bleeder" and had by an unaffected husband 2 bleeding 
sons and 1 bleeding daughter besides 4 others who died of scarlatina. Her brother, IV, 8, 
had a daughter, V, 5, who was a bleeder until 15, and then died of a hemorrhage of the lungs 
consequent upon tuberculosis. There were other children all of whom died young of 
scarlatina. The normal brother, IV, 10, had 12 normal children. The next 2 had no 
offspring. The youngest son, IV, 14, began to bleed while an infant, grew worse until he 
was 25 and has since improved. He married a cousin who is also a bleeder and they have 
6 children. Three of the daughters have not bled as yet. V, 9, has been a bleeder since he 
was 8 months old and bleeds until he faints; V, 10, has been a bleeder since she was 8 
months old and V, 11, bleeds occasionally but not very severely. (After C. B. Davenport.) 



inheritance as sex-linked factors do in the fruit fly. A family history 
showing the inheritance of haemophilia is outlined in Fig. 203. There 
is an analogous defect in the horse. In man a number of other defects 
have been traced some of which display a Mendelian type of behavior, 
and a surprisingly large number of them are sex-linked. 

Thus among dominant defects in man Guyer lists achondroplasy , 
abnormally short limbs along with normal head and body; keratosis, 
thickening of the epidermis; epidermolysis, excessive formation of 
blisters; hypotrichosis, a hairless and toothless condition; diabetes 



DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 525 

insipidus; diabetes mellitus ; muscular atropJuj of the ordinary type; 
glaucoma, internal swelling and pressure on the eyeball; displaced 
lens; colomha, an open suture in the iris; piehaldness, spotting of the dermal 
coat; corneal opacity ; Huntington's chorea, a disease similar to St. Vitus' 
dance, a dangerous malady which first exhibits itself in middle life; 
retinosa pigmentosa, pigmentary degeneration of the retina; polydac- 
tylism, extra fingers and toes; syndactylism, fusion of digits; congenital 
cataract; hemeralopia, hereditary night-blindness; and hrachydactylism, 
shortening of the digits. A corresponding list of recessive defects includes 
susceptibility to cancer; chorea, St. Vitus' dance; true dwarfism in which 
all parts are proportionally reduced; alkaptonuria, Urine darkens after 
passage; alcoholism and criminality when based on mental deficiency, 
hereditary hysteria; multiple sclerosis, diffuse degeneration of nervous 
tissue; Friedreich's disease, degeneration of the upper part of the spinal 
cord; Meniere's disease, dizziness and roaring in the ears; Thomson's 
disease, lack of muscular tone ; hereditary ataxia; possibly the tendency 
to become hard of hearing in advanced age; possibly non-resistance to 
tuberculosis; feeble-mindedness of various types. In man the sex- 
linked defects include Daltonism, color-blindness characterized by 
inability to distinguish between reds and greens; hcemophiUa, ex- 
cessive bleeding from wounds; m|/op?a, near-sightedness; multiple sclerosis; 
neuritis optica, progressive atrophy of the optic nerve; Gower's muscular 
atrophy; night-blindness, some forms ; ichthyosis, a peculiar scaly condition 
of the skin; syndactyly, some forms. 

When the short time in which true systematic attention has been 
given to the inheritance of human defects is recalled, it can be seen 
that this even now incomplete list is truly a formidable array of infirm - 
ties. Commonly of course prejudice against those individuals which 
display noticeable defects is so great that they have less opportunity 
for reproduction than the more fit members of society. This is particu- 
larly true of the more serious, dominant defects, and there is a consequent 
tendency for these to run out. Some very serious dominant defects, how- 
ever, such as Huntington's chorea are commonly not exhibited until 
late in life after the common reproductive period is passed. It is 
difficult, therefore, to state in such cases which individuals should and 
which should not be permitted to bring forth offspring. In the case of 
recessive defects, the factor may be carried by normal individuals, and 
may therefore continually crop out among the progeny of normal parents. 
To breed out defects in general, therefore, it is necessary to reject all 
defectives for breeding purposes, and to mate all individuals from de- 
fective stock, if at all, to stocks known not to be defective with respect 
to the infirmity in question. A word of warning is necessary here on 
account of the nature of our knowledge of human heredity. The 



526 GENETICS IN RELATION TO AGRICULTURE 

data which have been collected are of the observational type, the conclu- 
sions which have been drawn are inferential, and many of them are 
still in dispute among skilled investigators, nevertheless the results 
which have been reached have a high degree of probable validity. 
Whether it is desirable from a sociological standpoint to reject all de- 
fective individuals from reproduction, and to exclude in addition those 
individ vials which while themselves normal may transmit some defect, 
is a grave question which should be considered very carefully. It is 
not, however, a question of genetics, the geneticist can only point out the 
remedy. It is a matter of grave concern to any nation that 30 per 
cent, of its population should carry hereditary taints, and yet that is 
about the proportion which Rosanoff estimates on the basis of data 
from certain localities in the United States. 

An example nearer home might be taken, but that of the cretins of 
Aosta described by Whymper is perhaps one of the most striking cases 
of the baneful effects which may follow an improper social treatment of 
defectives. These horrible examples of human deformity, often goitrous 
and almost devoid of intelligence and common decency, had been the 
objects of such pity and charity on the part of society that the condition 
had actually been favored and preserved rather than weeded out. Under 
the old regime the cretins were given the best of care and attention, 
intermarriage between them was sanctioned by the church, and such a 
premium paid upon the deformity that it was multiplied from generation 
to generation. The disease was known to reappear in successive genera- 
tions, yet no attempt was made to stamp it out in the perfectly obvious 
way by preventing reproduction by cretins. Later, however, when this 
method of dealing with the problem was applied, the prevalence of 
cretinism was soon strikingly decreased. 

Whether or not the rate of reproduction in cretins is greater or less 
than that in normal stocks is a question. It is, however, known that 
many families which carry the taint of feeblemindedness are astonish- 
ingly prolific. None but a positive method, therefore, can deal with 
cases of this kind under modern conditions; for there is no assurance 
whatever that such families under present day conditions will tend to 
run out on account of differential survival when compared with normal 
stocks. The student who wishes to carry this subject further will find 
abundant confirmation of these statements in the records of the Juke 
family, the Nam family, the Hill folk, and the Kallikak family. 

Defects in Domestic Animals. — Curiously enough, if it be desired 
to trace the inheritance of defects in animals, man himself provides some 
of the most interesting and best investigated cases. The reason for 
this is very obvious; the animal breeder does not propagate his defectives, 
he rigidly culls them out of the herd. As a consequence although a few 



DISEASE AND RELATED I'flENOMENA IN ANIMAL BliEEDINa 527 

defectives occur from generation to generation in domestic animals, 
they are immediately condemned so far as breeding purposes are con- 
cerned. Little, therefore, is known concerning the inheritance of defects 
in animals except in rare cases where the defect may be of some use to 
man. We refer particularly to such characters as the polled condition 
in cattle, hornlessness in sheep, mule-foot in hogs, taillessness in cats, 
and like characters. Among them we might also include the famous 
Ancon sheep, now extinct. A case of an extremely malformed, defective 
condition is that reported by J. Wilson in Dexter-Kerry cattle. These 
cattle occasionally produce calves which are monstrous and live only a 
few hours, but they all conform to a definite type. Wilson describes 
them thus. "The body is short and stout; the upper jaw is short, giving 
the head a bulldog appearance; the legs are extremely short, being little 
more than a finger-length; the tail arises from well up the back; and the 
ventral skin is unclosed so that the intestines protrude." Apparently 
in this monstrosity we have a simple factor difference from the normal 
form of such a nature as to lead to total incapacity for independent 
existence. This condition is almost certainly the outcome of matings 
of normal individuals heterozygous for a defective recessive factor. 
The only moral that need be pointed out here is that a surprisingl}^ large 
proportion of defective conditions are heritable. The animal breeder 
is, therefore, fully justified in avoiding so far as lies within his power 
breeding froni defectives or even from normal individuals belonging to 
defective stocks. 

Immunity to Disease. — Animals may exhibit different sorts of im- 
munity to disease. Thus there is a certain kind of racial immunity 
which is just as characteristic of a given race as its morphological charac- 
ters are. Fowl cholera and foot-and-mouth disease do not affect men. 
Apparently the degree of relationship may be even much closer. Thus 
according to Tyzzer susceptibility to transplantable tumors varies in 
different strains of mice. Two strains of common mice, one from Buffalo, 
N. Y., and the other from Providence, R. I., and a strain of Japanese 
waltzing mice were used in the experiments. Although the investigations 
were not carried on extensively enough to he conclusive, they do indi- 
cate very definitely different degrees of susceptibility to various kinds of 
tumors. It was found that the Ehrlich tumor developed in 30 per cent, 
of the Providence mice and in 60 per cent, of the Buffalo mice. It became 
established in the Japanese mice, but practically failed to develop. The 
Jensen tumor developed in 40 per cent, of the Providence mice, but failed 
to develop at all in the Buffalo and Japanese mice. A Japanese type of 
tumor developed in all but three out of 145 Japanese individuals which 
were inoculated, but failed to develop at all in common mice. In the 
zebu we appear to have an analagous condition, for according to Pucci 



528 GENETICS IN RELATION TO AGRICULTURE 

a male and female Gujarat zebu among seventy brown Alpine cows were 
the only individuals spared by the foot-and-mouth disease. 

There is another type of immunity which is characteristic of certain 
individuals within a race. It is a matter of common observation that 
individuals occasionally appear which are completely immune to a given 
disease. It is difficult to state precisely upon what this immunity 
depends, but it is none the less definite, and it is apparently often heritable. 
When heritable it may under appropriate conditions become a racial 
character. It has been thought that upon this depends the comparative 
immunity which certain races bear against given diseases. The negro of 
the West Indies is comparatively immune to the ravages of yellow fever, 
presumably because for centuries the more susceptible individuals have 
succumbed to the disease, so that the race has been propagated for the 
most part by less susceptible individuals or those which survived the 
disease. The white man on the other hand is more susceptible to yellow 
fever because no process of selection has weeded out susceptible strains. 
Measles, also, is considered a very mild disease among Caucasian peoples, 
but among the North American Indians it is very severe, spreading 
through tribes like a veritable plague and proving fatal in many cases. 

There is another type of immunity which is acquired by the individual 
during life. Persons who have had smallpox have had conferred upon 
them an immunity which lasts for several years, and the same is true 
of other diseases in man and in other animals. This type of immunity 
may be induced artificially in the individual by appropriate treatment 
such as is done in vaccination, the administration of anitoxins and other 
forms of immunization. In animals the practice is seen in the distribu- 
tion by experiment stations of blackleg vaccine for calves and hog cholera 
serum for swine plague. 

A type of indirect immunity is that of resistance to attack by agents 
carrying a particular disease. Thus yellow fever is carried by a certain 
kind of mosquito, Aedes calopiis, and malaria by certain species of the 
genus Anopheles. It would be possible, therefore, for individuals to 
enjoy freedom from the attacks of either of these two diseases if they hap- 
pened to be resistant or repellent to the attacks of the particular mos- 
quitos which carry the disease. Sometimes active immunity is associated 
with such resistance to attack. This matter does not look so strange 
when it is recalled that often a very specific relation exists between 
parasite and host among animals, and that very often diseases are trans- 
mitted by insects and other animal pests. In animals immunity of this 
kind is exhibited by the zebu against the Texas fever tick. According 
to Mohler the immunity which the zebu enjoys to tick infestation depends 
upon three factors; the sebum secreted by the glands of the skin, which 
has a repellent odor repugnant to insect life; the toughness of the skin. 



DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 529 

which, although as thin as that of domestic cattle, is more difficult to 
pierce; and finally the short coat, which does not provide shelter for 
the ticks. The bison also appears to be immune to the attacks of the 
same pest. 

Breeding for Immunity. — The matter of breeding for immunity to 
disease is one which may have tremendous practical importance in animal 
breeding. Success in such breeding depends upon the existence of strains 
or races of immune animals, and upon the transmission of these characters 
to the offspring. Thus in Tyzzer's experiments with mice it was found 
that the Ehrlich tumor, which made only a very insignificant growth in 
Japanese mice, also failed to develop in the Fi hybrids between common 
and Japanese mice. In the same fashion Fi mice of the cross common X 
Japanese proved to be resistant to inoculation with the Jensen tumor, 
in this respect resembling the Japanese parent. On the other hand Fi 
mice of the cross common X Japanese were very susceptible to inocula- 
tion with the Japanese tumor, even more so than the Japanese mice 
themselves. Here the immunitj^ of common mice to inoculation does 
not appear to have been carried over to the hybrids. The Fo of this 
cross, however, behaved very peculiarly. Of fifty-four F2 individuals 
not one proved susceptible to the tumor, and sixteen F3 individuals 
gave like negative results. The transmission of immunity to disease is 
established by these experiments, but the exact factor relations cannot 
be stated. 

In domestic animals the possibility of breeding for disease resistance 
has long been held in mind, and in some cases steps have actually been 
taken in that direction. In the Southern States, particularly in Texas, 
Texas fever has annually levied its millions of dollars' tribute upon the 
cattle industry. The trouble has apparently been intensified since the 
attempt to grade up range cattle by the use of pure-bred Hereford and 
Shorthorn bulls, for these improved types of Northern cattle are more 
susceptible to tick infestation and splenetic fever than their hardier, 
but otherwise less desirable, range relatives. Apparently there are some 
grounds for the belief that range cattle have received infusions of zebu 
blood from early Spanish importations into Mexico and from zebu cattle 
brought to South CaroUna in 1849. Accordingly it is not impossible 
that the comparative freedom from tick and insect infestation which is 
characteristic of unimproved range cattle, has come originally from the 
zebu. 

As in our own domestic cattle so in the zebu there are many different 
breeds, and they present differences no less striking than those of the 
Northern cattle. They have been bred for centuries under the tropical 
conditions of India and Africa. They are disease resistant; they are 
able to withstand tick and insect pests to which the Northern breeds 
34 



530 



GENETICS IN RELATION TO AGRICULTURE 



of cattle succumb; and they can endure great heat; but they suffer from 
low temperatures (Figs. 204 and 205). 




Fig. 204. 



-A Gujarat zebu bull, a splendid specimen of the breed. Imported from India 
and used as a herd bull in Brazil. (From the Journal of Heredity.) 




Fig. 205.- — A Gujarat zebu cow. Imported from India and considered an unusually 
superior specimen of the breed. {From the Journal of Heredity.) 



The zebu may be used in two different ways in animal breeding; 
either in obtaining direct hybrids with native cattle, or in establishing 



DISEASE AND RELATED PHENOMENA IN ANIMAL BHEEDING 531 

new breeds. The foniuM- manner of utilization is relatively simple — 
it requires merely that herds of both pure-bred cattle and zebus be main- 
tained; the latter mode of employment necessitates long continued selec- 
tion before stability of characters will have been reached. 

If zebu cattle are to be used for obtaining cross-bred commercial 
stock, it is necessary that the immediate hybritls possess superior charac- 
ters for such purposes. This matter appears to be in some doubt, but 
discounting the opinion of those who are Wind partisans of the Northern 
breeds, it appears that zebu-Northern crosses unite many of the favorable 
characters of both breeds. The cross is very easily made. Thus Pucci 



^^■^IP 


r?^ 




^vl 


Y^ 


^^^^^^V^^^L B 




^3f «»• 



Fig. 206. — An Fi zebu-Hereford heifer. The Hereford characters appear to be predom- 
inant. {From the Journal of Heredity.) 

reports upon the results of using a Gujarat bull on cows of the Maremma, 
Romagnola, and Perugia breeds in experiments conducted at the Perugia 
Institute of Animal Husbandry in Italy. Of 113 cows used only nine 
failed to give calves, and these nine were also barren when mated with 
hulls of their own breeds. 

With respect to the characters of the Fi offspring there seems to be a 
considerable difference of opinion. Pucci who has apparently studied 
the matter most thoroughlj^ states that zebu characters are dominant 
with respect to fineness of skeleton, abundance of dewlap, development 
of ear, slope of rump, and general muscular development. It appears 
to be generally agreed that the crosses are more vigorous and growthy 
than the parents; according to early reports, 'they exceeded either parent 
l)y 50 per cent, in this respect. According to Nabours the predominant 
appearance of zebu characters in the Fi offspring occurs only when range 
cattle are used. He ascribes this fact to the presence in range cattle of 
a strong infusion of zebu blood. He found that when crossed with 



532 GENETICS IN RELATION TO AGRICULTURE 

Shorthorn or Hereford cattle, the hybrids partook most of the characters 
of the Northern breed, save for the shght hump and greater development 
of dewlap and sheath, which are characteristic of the zebu. This ob- 
servation is borne out by the zebu-Hereford hybrid shown in Fig. 206. 
With respect to market features, the hybrids appear to be held in high 
regard in regions where the cross has been made in great numbers. In 
Tunisia and Brazil at least butchers prefer them and are willing to paj'- 
a premium to get them. 

With regard to resistance to disease the zebu appears to transmit 
most of its qualities, at least in some degree, to the Fi offspring. They 
are resistant to foot-and-mouth disease, anthrax, and splenetic fever. 
They withstand the heat of tropical climates, and the insect pests which 
thrive in such places. According to several notes they are not infested 
by ticks, but this lack of infestation does not appear to be associated 
with any definite immunity to Texas fever itself. On the contrary one 
note states explicitly that both the pure-bred zebu and the Fi hybrids 
are susceptible to infection with the Texas fever protozoon, but their 
high resistance to the disease keeps it from becoming as serious with 
them as with Northern cattle. 

Apparently both male and female hybrids are fertile, in which respect 
they appear to differ from most other species hybrids among the Bovidae, 
the females of which are commonly fertile while the males are sterile. 
There is evidence of Mendelian inheritance when specific characters are 
considered, but coupling and physiological relations appear to exert a 
considerable influence on the segregation and expression of characters. 
Evidence of this comes from the fact that so many of the F2 hybrids so 
closely approach to an expression of the sum total of characters of either 
the zebu or the Northern breed as to be indistinguishable from them. 
In Brazil free interbreeding in hybrid herds apparently leads to a pre- 
dominance of zebu characters in a verj' few generations, perhaps on 
account of the greater prolificacy of the zebu. The increased size and 
vigor of the Fi is not maintained, and constancy of blended characters 
has apparently not been attained in any case. Tick resistance at least 
appears to be carried over to subsequent generations, although unfor- 
fortunately the records are not satisfactory. 

From a practical standpoint, therefore, it appears that the zebu might 
be used as a means of combating tick fever in the Southern cattle district. 
Whctlier the method would prove desirable in view of the perfection 
attained by other methods of ridding pastures of ticks is, however, ques- 
tionable. If utilized, however, it appears that the method should be 
that of continually mating zebu bulls with range cows. In this fashion 
it is possible to secure animals free from disease and at the same time of 
very superior market qualities. This would necessitate a practice like 



DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 533 

that in crossbreeding, namely of keeping pure-bred herds of both zebu 
and Northern cattle and continually crossing them. Central herds of 
zebus could easily be established to supply the demand for zebu bulls. 
Zebu bulls appear to be more prolific than Northern bulls, for it is possible 
with them to count upon an average of about sixty calves per year from 
each bull. The utilization of the hybrids for the establishment of a new 
breed is another matter, and one requiring long time and careful attention. 
It should, however, be carried on under competent guidance such as 
might be afforded by experiment station supervision. 




Fig. 207. — Bison btil 



Bison americanus. (Courtesy National Zoological Park, Wash- 
ington, D. C.) 



The other hj-brid which gives some promise of use in American 
agriculture is that obtained by crossing the bison and domestic cattle. 
Reciprocal hj-brids may be obtained, but apparently it is easier to get 
hybrids b}' using a bison bull on domestic cows. This hybrid represents 
a more violent cross than the zebu hybrid, inasmuch as considerable 
difficulty attends the production of hybrid progeny. Cows very often 
die while calving, apparently on account of the physical difficulties 
involved in giving birth to the hybrids, which possess a considerable 
hump like that on the bison. The hybrids are intermediate between the 
parents in respect to most characters. In vigor and size they are su- 
perior to either parent; from the market standpoint both with respect 
to flesh and coat they are very desirable animals. Practically all the 
animals which have been born have been females, and these are fertile, A 



534 



GENETICS IN RELATION TO AGRICULTURE 



few males have been born and these have always proved sterile. The 
hybrids are apparently very uniform. The female hybrids appear to have 
no difficulty in parturition when bred to domestic bulls or bison bulls. 




Fig. 208. — An Fi hybrid between a bison bull and an Aberdeen-Angus cow. 
character. {From Journal of Heredity.) 



Note polled 




Fig. 209. — Quinto Porto, five-eighths buffalo, three-eighths polled Hereford. A fertile bull 
which displays the characters of both species. {From the Journal of Heredity.) 

When bred to bison bulls, the three-fourths hybrids which are produced 
are intermediate between the bison and the hybrid, and one-fourth 
hybrids produced by breeding an Fi female to a domestic bull are inter- 
mediate between the hybrid and domestic cattle. Bulls of these two 



DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 535 

grades are sometimes fertile. The bull Quinto Porto which is shown in 
Fig. 209 has proven fertile with Hereford cows. His pedigree is as 
follows : 

f pure bison bull 

Quinto Porto 1 

Fertile hybrid bull j i pure-bred Hereford bull 

l hybrid cow \ f pure bison bull 

[ hybrid cow \ 

[ pure-bred Hereford cow 
Fig. 210. — Ancestry of a fertile hybrid (rtitttilo bull. 

By long-continued selection it would be possible apparently to transfer 
many of the excellent qualities of the bison such as superior coat, greater 
hardiness, resistance to tick and insect infestation, and superior beef 
qualities to domestic cattle. There is no possibility of utilizing it like 
the zebu for securing croiss-bred animals of superior qualities, not because 
of lack of such superior qualities, but because of the difficulties of secur- 
ing Fi hybrids. For range conditions it might be possible to establish 
a breed of cattalo which would have many advantages. Here again the 
need of continuous attention and encouragement such as might be given 
bj^ experiment station supervision would be a valuable aid toward such 
a goal. 



CHAPTER XXXIV 
SEX IN ANIMALS 

Sex-determination with its attendant problems has always been a 
subject of great interest to practical animal breeders; and the art of 
breeding has not lacked rules by which the sex ratio might be shifted in 
various ways to the advantage of the breeder. But most of these rules, 
like many beliefs current at one time or another in animal breeding have 
been founded upon inadequate evidence or unsound reasoning. Sex 
remains a matter beyond the control of the breeder: its ultimate control 
is entirely problematical. 

The Determination of Sex. — The thesis that sex is determined at the 
time of fertilization has been elaborated fully in Chapter XI. It was 
pointed out there that sex, like other characters of the individual, has 
a definite factorial basis, that the factorial constitution of the individual 
with respect to sex as well as to other characters is fixed by the constitu- 
tion of the two gametes which unite to form the zygote. There is 
every reason to believe that sex is determined in this same fashion in 
domestic animals, at the time of fertilization; and that any treatment 
subsequent to that time cannot affect the sex of the individual. At least 
this much may be said, that any theory of sex-determination in the higher 
animals which is based upon other factors than chromosome constitution, 
must be brought into harmony with the known facts of the chromosome 
relations in sex-determination. 

Sex-determination in Mammals. — It appears to be fairly well estab- 
lished that the inheritance of sex in mammals always is of the XY type, 
that is the females are homozygous for a determiner of femaleness whereas 
the males are heterozygous. Since this group includes practically all 
domestic animals, except the feathered ones, it follows that in horses, 
cattle, sheep, goats, swine, etc., the mode of inheritance of sex is of this 
type. The direct evidence for this conclusion in domestic animals is 
exceedingly meager, but the main outlines are sufficiently clear to provide 
fairly satisfactory confirmation of this general conclusion. 

For direct cytological evidence of the mode of determination of sex 
in these domestic animals we are indebted particularly to the extensive 
investigations of Wodsedalek. These investigations do not provide a 
complete body of evidence, but they indicate very strongly that unequal 
distribution of chromosomes takes place in the male in the horse and 

536 



SEX IN ANIMALS 



537 



swine. Wodsedalek finds that in the horse the somatic nvinilMT of 
chromosomes in the female is thirty-eight, in the male thirty-six. In 
Fig. 211 arc shown stages of the heterotypic division from which these 
conclusions are drawn. There are apparently two accessory chromo- 
somes, and these both go to the same pole. Consequently half the sper- 



/:.;'• 










Fig. 211.- 
somes are 



-Three figures illustrating heterotypic division in the horse. The scx-chromo- 
seen slightly separated from the autosomes in each figure. {After Wodsedalek.) 



matozoa contain nineteen and half seventeen chromosomes. No direct 
cytological evidence of gametogenesis in the female has been obtained, 
it is merely assumed that those gametes containing nineteen chromosomes 
are normally female producing and that those which contain seventeen 
are male producing. In the pig very much the same state of affairs exists. 



,.,,C'5>" 



f0^^:^ 



t^ 









Fig. 212. — Three figures illustrating stages in the heterotypic division in the male pig. 
The sex chromosomes are seen passing to one pole ahead of the autosomes in the middle 
figure. {After Wodsedalek.) 



The somatic number of chromosomes in the female is twenty, in the male 
eighteen. In the heterotypic division in the male, stages of which are 
shown in Fig. 212, the two accessory chromosomes pass to the same 
pole. In consequence in the male half the gametes contain ten and half 
eight chromosomes. From this the conclusion is drawn without further 
evidence that the male is heterozygous and the female homozygous for 



538 GENETICS IN RELATION TO AGRICULTURE 

the sex-factors. In man cytological investigations have been made 
by Guyer, Montgomery, von Winiwarter, Wiemen and Evans. ^ The 
evidence is very conflicting as regards total chromosome number, but it 
is not at all in conflict with the hypothesis of an X Y type of sex inheri- 
tance in man. Von Winiwarter finds that there are forty-seven chromo- 
somes in the male. In the formation of spermatozoa he observed that 
half received twenty-three and half twenty-four chromosomes. In the 
female his evidence pointed to forty-eight as the somatic number, con- 
sequently it may be assumed that all egg-cells normally contain twenty- 
four chromosomes. The observations of Guyer and Montgomery which 
were made with material from the negro, seem to indicate that the chro- 
mosome number in this race is one-half that in the white race. This 
evidence also indicated that a pair of accessory chromosomes exist in the 
male, but the evidence is not conclusive. The existence of an X and a Y 
chromosome in the male has been claimed by Wieman. The cytological 
evidence, therefore, so far as it goes, indicates that there is nothing in 
the internal mechanism of mammals in conflict with the belief that the 
female is homozygous for a sex-factor, and the male heterozygous. 

The evidence from hereditary phenomena is not very extensive for 
domestic mammals. In man, however, as has been pointed out in other 
places, a number of sex-linked characters are known, and these follow 
the XY type of sex-determination. Here the evidence is strong enough 
to be conclusive, but in no domsetic animal with the exception of the cat 
is any sex-linked character known. In the cat the characters in question 
are black, orange, and tortoise-shell coat colors. According to Ibsen's 
analysis of the case, which appears to be most satisfactory, there are two 
factors giving the following color classes and formulae: 



Females 


Males 


Classes 


{BTX){BTX) 


{BTX)Y 


Tortoise shell 


{BtX){BlX) 


(BtX) Y 


Black 


{bTX){hTX) 


{bTX)Y 


Orange . 


{htX) (htX) 


(btX) Y 


Orange 



The experimental evidence as to the relation of the colors has been dis- 
cussed by Doncaster, Little, Whiting, and Ibsen, but the analysis is still 
in debate. Such as it is, however, there is no room for doubt that sex- 
linked characters occur in the cat and are distributed in accordance with 
the accepted conclusion that the male is heterozygous for sex and the 
female homozygous. 

1 Dr. Herbert M. Evans of the University of California. Data not published, but 
in the case of a white man hundreds of counts give constantly 48 chromosomes in the 
spermatogonia. If the number in the white male is 48, instead of 47 as von Wini- 
warter concluded, it would indicate the existence of a F element as well as one X 
element in the male. 



SEX IN ANIMALS 



539 



Sex-determination in Birds. — Contrary to the condition in mammals, 
in l)irds the determination of sex appears to depend upon the WZ type of 
sex-inheritance, for in them the female is heterozysons for the sex factor, 
and the male homozygous. The cytological evidence for this conclusion, 
while limited to investigations on the domestic fowl, has through the 
persistent and painstaking efforts of Guyer, been very definitely if not 
quite completely worked out. In this species there arc 18 chromosomes 
in the somatic and primary germ cells of the male and 17 in the female; 
in the former there are two sex-chromosomes and in the latter, one. 
The eggs, therefore, are of two classes, those possessing a sex-chromosome 
and hence male-producing, and those lacking it and hence female-pro- 
ducing. In the nuile both the sex-chromosomes pass together during the 
reduction division into one daughter cell, so that half of the sperm are 
provided with a sex-chromosome and half are not. The fate of the 
sperm lacking a sex-chromosome is still somewhat in doubt, but there is 
considerable cytological evidence indicating that part of the developing 
sperm degenerate. Moreover, the fact that statistical examination of 
head length of the spermatozoa reveals only one class of sperm certainly 
strengthens this inference. Thus the cytological evidence is in complete 
liarmony with the extensive evidence from sex-linked characters in this 
species. 

The mode of inheritance of the barring factor in the domestic fowl 
has already been described in detail. Other factors in the domestic fowl 
which have been found to exhibit sex-linked inheritance are those for 
silky pigmentation, for inhibition of red in the plumage of the Columbian 
Wyandotte, gray in the White Wyandotte, and a factor for high fecundity. 
In pigeons, also, it has been found that the factor for dilute pigmentation 
is sex-linked, and follows the same type of distribution as that described 
for the domestic fowl. 

The Sex-ratio. — The Mendelian theory of the inheritance of sex 
stipulates that in the heterozygous sex male and female producing 
gametes are formed in equal numbers; for in every reduction division one 
pole of necessity must con- 
tain the accessory chromo- T'^kle LXX. Sex Ratios in Animals, 
some or sex-determining 
factor whereas the other 
must either lack it or con- 
tain its unequal homologous 
chromosome or sex-deter- 
mining factor. As a con- 
sequence of this fact we 
should expect the sex-ratio 
to show an approximate 



Animal 



Mai 



Females 



Authority 



Horse 


98.3 


100 


Diising 


Cattle .... 


107.3 


100 


Wile kens 


Sheep 


97.7 


100 


Darwin 


Swine 


111.8 


100 


W'ilckens 


Rat 


10.5.0 


100 


Cuenot 


Dove 


105.0 


100 


Cuenot 


Fowl 


94.7 


KM) 


Darwin 



540 GENETICS IN RELATION TO AGRICULTURE 

equality of males and females in every generation. Extensive statistical in- 
vestigations of the sex-ratio in a number of animals demonstrate, however, 
that there are usually small but significant deviations from the numerical 
equality of the sexes. The data in Table LXX collected by Morgan 
illustrate this point. 

In man the sex-ratio varies among different peoples, but almost in- 
variably the proportion of males is slightly in excess of that of females. 




10.2510.75 ll.-.'5 11.75 12.25 12.75 13.25 13.75 14.25 14.75 15.25 15.75 

Fig. 213. — Frequency polygon of head lengths of spermatozoa in the pig. (Data from 

Wodsedalek.) 

In dealing with sex-ratios it must be emphasized that a study of the 
deviations which are obtained may point to conditions which alter the 
sex-ratio without disturbing the mode of determination of sex. The 
mechanism of sex-determination is not such a one as would lead invari- 
ably to numerical equality of the sexes. This may easily be seen by 
a consideration of several disturbing factors which may enter into it. 

The first of these has to do with the relative sizes of male and female 
producing gametes, particularly in those animals in which the male is the 
heterozygous sex. Thus Wodsedalek has shown for the pig and the horse 
that there are two intergrading classes of spermatozoa as respects size 
(Figs. 213 and 214). Unquestionably in both cases the larger class 



SEX IN ANIMALS 541 

are those which contain the accessory chromosome or chromosomes. 
The larger class, therefore, includes the female-producing spermatozoa, 
the smaller class the male-producing spermatozoa. If this difference in 
size should be correlated with a slight difference in activity, it is con- 




S.50 8.75 3.00 9.25 9.50 9.75 10.00 10.2510.50 10.75 11.00 11.25 11.5011.7512.0012.25 

Fig. 214. — Frequency polygon of head lengths of spermatozoa in the horse. 
(Data from Wodsedalek.) 

ceivable that chance would favor fertihzation with the more active 
spermatozoa, and would produce a consequent disturbance in the 
sex-ratio. 

Moreover, the gametes of the heterozygous sex, since they are made 



542 



GENETICS IN RELATION TO AGRICULTURE 



up of two classes having different chromatin contents might display dif- 
ferential mortality under the stress of slightly unfavorable conditions. It 
may easily be seen, therefore, that shght or conceivably wide variations 
in the sex-ratio may be due to the operation of factors other than those 
which actually determine sex. 

Causes of Unusual Sex-ratios. — In this discussion we desire to 
treat only of causes of sex-ratio disturbance which may operate in the 
higher domestic animals. The curious and interesting conditions which 
are found in some insects and other lower animal forms will not be men- 
tioned save in so far as they may tlirow light upon the problem of sex- 
ratio disturbance in higher animals. 

In some cases hybridity appears to favor a disproportionate produc- 
tion of one sex or the other. Thus in the bison-cattle crosses, the 

production of males is rel- 



Table LXXI. — Ratio of Sexes in H'i'BRiD 
Guinea-pigs {After Detlefsen) 



Generation 


Males 


Females 


Total 


Number of 

males to 100 

females 


F„}4wM 


14 


23 


37 


60.9 


Fi, U wild 


.31 


52 


83 


59.6 


Fz,H wild... 


101 


116 


217 


87.1 


F„ He wild... 


159 


153 


312 


103.9 


n, H2wild... 


173 


171 


344 


101.2 


Fs, Hi wild... 


58 


64 


122 


90.6 


Fi, H28 wild. . 


16 


21 


37 


76.2 


Totals 


552 


600 


1152 


92.0 



ativcly rare. This, how- 
ever, appears to be due to 
the fact that physical 
difficulties make it prac- 
tically impossible for a 
cow to bear to maturity a 
bison-cattle fetus and to 
give birth to it. There are 
two reasons for this, the 
increased size of the hybrid 
fetus, and the development 
of a large hump which 
cannot be accommodated 
by the normal pelvic con- 
formation in the cow. Consequently the fact that practically all 
the animals born of this species cross are females, may simply 
be due to abortion and death of male fetuses. The amount of 
trouble is sufficient in this case to give room for a potential equality 
of sexes in such crosses. Detlefsen reports similar results from a cavy 
species cross, which gave a disproportionately low ratio of males. The 
data are given in Table LXXI. The earlier generations show a con- 
siderable deviation from the normal equality of the sexes. With suc- 
cessive back-crosses, however, the ratio soon becomes one of practical 
equality. 

When races are more closely related a different result is produced. 
Thus Miss King found in crosses between wild and albino rats a sex-ratio 
of 119.1 males to 100 females among 425 hybrids representing the 
first three hybrid generations of such a cross. Among guinea-pigs 
crossed inter se, Minot found among 410 individuals a sex-ratio almost 



SEX IN ANIMALS 



543 



exactly the same as that among Miss King's rats, 119.2 males to 100 
females. 

Among wide crosses of hybrid birds the percentage of males also 
appears to be al)normally high, but this can hardly be taken as in 
conformity with the data of King and Minot, because in birds the mode 
of sex-inheritance is exactly the reverse of that in mammals. Ac- 
cordingly these data should be regarded as confirmatory of the data 
from bison-cattle crosses, and those of Detlefsen with cavy species 
crosses. 

As another possible source of variation in the sex-ratio, mention 
must be made of the time of service with respect to the inception of the 
period of heat. Various theories of sex have from time to time been 
founded on heat relations, some maintaining that the products of con- 
ception in early heat were more often males, others that they were more 
often females. Pearl and Parshley have made an experimental-statisti- 
cal study of this question, the data of which are given in Table LXXII. 
These data were collected from farmers in the state of Maine, and 
represent all breeds and ages of cattle. Pearl and Parshley draw 
attention to the fact that they show a steady increase in the proportion 
of male births with later coitus. The question as to the general signifi- 
cance of this fact, they discuss at some length. In Table LXXIII is 
given the statistical treatment of these data. Only in one case, that 

Table LXXII. — The Effect of Service at Different Periods of Heat on the 
Sex-ratio in Cattle {Data of Pearl and Parshley) 





Total 
offspring 


Sex of offspring 


Percentages 


Males to 


Time of c<jitu.s 


Males 


Females 


Male birtlis ! Female births 


100 females 


Early in heat 

Middle of heat 

Late in heat. ., 


248 
125 
107 


123 
67 
65 


125 

58 
42 


49.60 + 2.14 
53.60 + 3.01 
60.75 ± 3.19 


50.40 
46.40 
39.25 


98.4 

115.5 
154.8 


Totals, all periods . . . 


480 


255 


225 


53.13 ± 1.54 


46.87 


113.3 



Table LXXIII. — Statistical Treatment of the Sex-ratios of T.\ble LXXII 
(Data of Pearl and Parshley) 



Groups compared 


Difference 


Odds against chance 
occurrence 


Percentage of male births 
Late in heat and early in heat 


11.15 + 3.84 
7.15 + 4.39 
4.00+3.69 


18.80 to 1 


Late in heat and middle of heat 


2.69 to 1 


Middle of heat and early in heat 


1.15 to 1 



544 GENETICS IN RELATION TO AGRICULTURE 

difference between the sex-ratio late in heat and early in heat can we 
place any confidence in the significance of the observed differences. In 
this case the odds are practically 19 to 1 against the chance occur- 
rence of the deviation : the question thus presents itself for consideration, 
shall we regard this difference merely as a chance occurrence or as 
possessing real significance? We are informed that more recent data 
(unpublished) according to Pearl make the relation of time of service 
to sex extremely doubtful. Moreover, Pearl and Salaman have shown 
that this relation does not hold for man. 

Metabolic Theories of Sex-determination.^ — ^Among other theories 
of sex-determination which have been advanced that of Geddes and Thom- 
son still provokes a considerable amount of speculation and discussion. 
According to this theory there is no sex-determinant or factor at all, in 
the strictly Mendelian sense of the word, but rather differences in meta- 
bolic relations lead to the production of different sexes. The theory is 
based on the conception that deep constitutional differences in metabol- 
ism-rythms exist between the two sexes; the male being characterized by a 
preponderance of katabolic or disruptive processes, whereas the female is 
distinguished by the emphasis placed upon constructive or anabolic 
processes. 

It should be pointed out, and indeed Geddes and Thomson fully ap- 
preciate this fact, that this theory is a physiological theory of sex-deter- 
mination, whereas the Mendelian theory is essentially morphological. 
The two are not necessarily in conflict with each other, so that an 
acceptance of the conclusion that differences in metabolic balance exist 
in the two sexes, does not necessarily imply rejection of the Mendelian 
theory of sex. It is only when supporters of this physiological doctrine 
argue for the possibility of sex-determination occurring after fertilization, 
or in other words, for reversal of sex as fixed at fertilization by the in- 
fluence of external factors operating subsequently to fertilization that 
difficulties between the two views arise. Thus to take a particular case, 
Pearl and Parshley on the basis of their statistical study of sex-deter- 
mination mention the possibility of changes occurring in the metabolic 
conditions in ova corresponding roughly to different periods in the time 
of heat, and that by virtue of these changes alterations of the sex-ratio 
might occur. But this suggestion carries with it reversal of potential sex 
as determined by chromosome relations at the time of fertilization. We 
may ask, therefore, with justice what sort of males are these which arise 
from such alteration of metabolic balance? Presumably staleness 
should not lead to extrusion from the egg of an entire chromosome, or 
should not in the light of present day conceptions of factor stability lead 
to changes in the factors of the sex determining chromosome. If, there- 
fore, staleness leads to a change in the relations between nucleoplasm and 



SEX IN ANIMALS 545 

cytoplasm, such males so far as nuclear material goes are potentially 
female, that is they possess the same nuclear material as do females. 
We do not argue that this is absolutely impossible, but what sort of germ 
cells do such males produce? From the morphological standpoint, it 
would appear that they would be all of one kind, instead of being of two 
kinds as is normally the case. Such males, therefore, when mated to 
females should give sex-ratios entirely dependent upon those changed 
metabolic relations which had been induced by early or late fertilization or 
some other cause — the entire morphological basis would be destroyed. 

Nevertheless, Riddle claims to have absolutely controlled se.x-determi- 
nation in the pigeon by experimental means. Having learned to identify 
the male and female producing ova, he was able, he says, to force either 
kind into the production of the opposite sex and he noted that the level of its 
metabolism was then shifted to the level characteristic of the germs of the 
opposite sex. Thus chromosomal correlation is here forced to failure but 
the metabolic correlation persists. Riddle infers, therefore, that the chromo- 
somal constitution is not an eflScient cause of sex; that it is but a sign or 
index and possibly an assistance in the normal maintenance of that which 
is essential — namely two different metabolic levels. 

Another case of sex reversal is the free- martin, the female of two-sexed 
twins in cattle, which has long been known to be perfectly sterile although 
rarely such females are perfectly normal. Lillie has found by a study of 
embryonic development that the phenomenon of sterility is due to fusion 
of the embryonic membranes of the twins and anastomosis of the blood- 
vessels, especiall}^ the arteries, so that there is literal community of blood 
during fetal life. If the anastomosis of the blood-vessels does not take 
place, the female is perfectly normal as is usual with the twins or multiple 
births of all other mammals. This fact, according to Lillie, can be explained 
only on the assumption that the fetal blood carries specific sex-hormones, 
because the only system of the female that is affected is the reproductive 
system. The male, on the other hand, is normal in all its parts, and this 
finds explanation in the fact that sexual differentiation of the male antedates 
by a little that of the female, and the development of female sex-hormones 
is probably inhibited from the start. From the study of extensive data on 
free-martins Lillie concludes that the female zygote must contain factors for 
both sexes, and that the primary determination of the female sex must 
therefore be due to dominance of the female factors over the male. If we 
think of this as a simple quantitative relation as Goldschmidt has done, we 
can explain the intersexual condition of the free-martin as due to an accel- 
eration or intensification of the male factors of the female zygote by the male 
hormones. The degree of the effect which is quite variable, as we have seen, 
would of course be subject to all quantitative variations of the hormone. 
Thus the case of the free-martin could come under the same general 

35 



546 GENETICS IN RELATION TO AGRICULTURE 

point of view as that of the intersexes of Lymantria according to Gold- 
schmidt with the one exception that the quantitative differences between 
the male and female factors of the female zygote necessary for the differen- 
tiation of female characters, are reduced in the free-martin by internal 
secretions instead of by variations of potency of the male factors in different 
varieties as in the intersexual hybrids of Lymantria." In attributing the 
free-martin condition to the male hormones Lillie means only to assert that 
they are the primary causes, and not that they are the decisive factors in 
each member of the series of events which result in the intersexual condition. 
He can, however, state confidently on the basis of present results that sex- 
determination in mammals is not irreversible predestination, and that with 
known methods and principles of physiology we can investigate the possible 
range of reversibility. 

It will be observed that neither of these cases invalidates the 
fundamental hypothesis that the sex-chromosomes are the normal differen- 
tiators in sex-determination. Moreover, the sex-chromosome hypothesis 
has this virtue, that it is based upon observed and firmly established 
differences between the sexes. It is disappointing in that it provides 
so little hope for control of the process, but our dissatisfaction with it 
from this standpoint should not close our minds to its superiority in 
definiteness and experimental evidence to all other theories of sex- 
determination. 

Inheritance of Unusual Sex-ratios. — From time to time reports are 
made of families both in man and other animals which appear to exhibit 
consistently abnormal sex-ratios. Families are reported in which male 
children only have been born for a number of generations, or in which 
only females have been born. Now according to the laws of chance such 
instances may occur occasionally without necessitating in any way the 
adoption of hypotheses subsidiary to that of the existence of a mechanism 
which potentially is calculated to give an approximate equality of the 
two sexes. But sometimes other factors do appear to be at work, and 
these may be mentioned briefly here. 

— The existence of sex-linked lethal factors in Drosophila has already 
been pointed out. Presumably these are factors which affect adversely 
the development or operation of some vital organ as a consequence of 
which individuals possessing the factor are doomed from the moment of 
conception to death at some stage in their life history. In some cases 
this occurs relatively late in the life history. Thus Bridges reports the 
discovery of a strain of flies with such a factor in which the morescent 
larvae are distinguished by the production of black specks within the 
body cavity. These larvae die when they reach maturity, but in other 
cases death must occur soon after fertilization. Moreover, in some cases 
the doomed individuals may occasionally overcome the defect and 
develop into normal adults. 



SEX IN ANIMALS 547 

The presence of one such factor leads to the appearance of a sex-ratio 
of 50 males to 100 females; if two sex-linked lethal factors occur in the 
same strain the ratio is less than 50 males to 100 females, the exact ratio 
depending upon linkage values of the two factors. Thus to take a particu- 
lar case, Lethal I is situated at locus 0.7 and Lethal III at locus 26.5 
in the sex chromosome. About 25 per cent, of crossing-over, therefore, 
occurs between these two factors. A female of the genetic constitution, 
(I1L3X) (LihX), therefore produces gametes In the following proportions: 

3 (/1L3X) :3 (L1/3X) :1 (L1L3X) :1 (hhX). 

When such a female is mated to a normal male, {L1L3X) Y, presumably- 
all the females survive because of the normal allelomorphs received from 
the sperm cell, but of the males only those which are of the genetic con- 
stitution {L1L3X) Y, therefore, only one in eight survive. This leads to 
a ratio of 12.5 males to 100 females in such a population. 

While this is a hypothetical case it may, however, be mentioned that 
Morgan and his associates have discovered at different times actual in- 
stances of two sex-linked lethal factors occurring in the same strain. 
Furthermore, the occurrence of sex-linked lethal factors, by virtue of 
their linkage relations with other factors is as well established as the exist- 
ence of any other Mendelian factors. They provide one means of ex- 
plaining definitely unusual sex-ratios without assuming any changes in 
the mechanistic relations. 

The inheritance of unusual sex-ratios in mammals and the reasons 
for such inheritance are in considerable doubt. About the only results 
which have been reported upon at all which give definite positive evidence 
are those of Miss King. These results have not been published in detail 
but a very preliminary note states that more than 22,000 albino rats have 
been reared in the course of the investigation. The experiments started 
with two pairs of rats from the same litter, and two lines of selection 
were made one for high proportion of males, the other for high proportion 
of females. The method of experimentation was rather unique. Thus 
the progeny of pair A , one of the original pairs were bred brother to sister 
without selection for six generations in order to establish a homogeneous 
race. After the sixth generation the brother to sister matings were 
continued but selection was invariably made from litters which had a 
high proportion of males. In Line B, the progeny of the other pair, the 
same procedure was followed except that selection after the sixth genera- 
tion was made from litters having a high proportion of females. Fifteen 
generations of selection in Strain A give a sex-ratio of about 125 males 
to 100 females; in Strain B a sex-ratio of 83 males to 100 females. This 
series of experiments seems, therefore, to indicate that unusual sex-ratios 
may be inherited. • Judgment, however, must be reserved until the com- 



548 GENETICS IN RELATION TO AGRICULTURE 

plete data are published, but we may again emphasize the fact that 
nothing in a disturbed sex-ratio need necessarily be taken to mean that 
the mode of sex-determination is anything other than that which we 
have stated in preceding portions of this chapter. 

Secondary Sexual Characters. — By a secondary sexual character 
is meant a character not immediately concerned with reproduction, 
but found only in one sex. In the more highly organized animals the 
differences between the secondary sexual characters of male and female 
are so great that by means of them alone it is possible to recognize in- 
stantly the sex of the individual. The secondary sexual characters 
include a wide variety of characters some of which are very definite and 
others are indefinite. Thus in most animals there are differences in 
size and general conformation, for example the stallion is larger and 
more rugged in build than the mare. The neck and forequarters are 
more fully developed than in the mare. These are differences of degree 
rather than kind. In certain breeds of sheep, however, horns are present 
in the males and absent in the females. In birds the differences between 
the sexes are often very striking, extreme instances of which are found 
among domesticated birds in such breeds as the Brown Leghorn fowl and 
the Rouen duck, and breeds of similar plumage coloration. 

The Nature of Secondary Sexual Characters. — Although secondary 
sexual characters are intimately related in expression to sex, yet careful 
distinction must be drawn between the factor basis of secondary sexual 
characters and that of sex-determination and sex-linkage. Sex-linked 
characters, of course, are those which are determined by factors borne 
by the sex-chromosomes. Such characters display peculiarities in he- 
redity which are dependent upon the chromosome relations, but the 
characters themselves may appear in either sex. Sex-determining 
factors are those which determine sex. Their presence results in profound 
effects upon the total developmental processes of the body. The different 
internal physiological conditions, therefore, which exist in the two sexes 
profoundly affect the reactions which the normal hereditary system 
exhibits in consequence of which many characters in the two sexes 
are different. These are the secondary sexual characters. In their 
development no difference need be postulated in the factorial basis 
save in the sex-factor itself. But the incentive to their development 
is found in the different internal conditions in the two sexes; the factors 
may be the same in both cases but these differences in internal condition 
lead to differences in the reaction products of those factors. Evidence 
of this view has been obtained largely from castration experiments. 

The Effects of Castration. — The effects of castration differ so much 
in different groups of animals that no general statement can be made 
which describes all these results. Among mammals the effects of cas- 



SEX IN ANIMALS 



549 



tration may be illustrated by a few typical eases. In sheep there are 
breeds which are hornless in both sexes, hornless in the female and 
horned in the male, and horned in both sexes. When both sexes are 
horned there are usually rather striking differences in the horns, 
those of the male being the better developed. Fig. 215 shows this for 
Dorset sheep which have been used in such experiments. The effect of 
castration in such breeds has been studied by Marshall. He finds that 
as a result of castration of males in those breeds in which males only are 
horned, the horns fail to develop. In breeds in which both sexes are 
horned castrated males develop horns like the females. 

The above effects are of interest when taken in connection with studies 
of inheritance of horns in sheep. Thus Wood has studied crosses between 




Fig. 215. — Dorset sheep showing the differences in development of horns in the two sexes. 

(After Shaw and Heller.) 

Dorset and Suffolk sheep. The latter are hornless in both sexes. The 
Fi of this cross consists of horned rams and hornless ewes. The F2 con- 
sists of horn and hornless individuals in both sexes in the approximate 
ratio : 

3 horned cf : 1 hornless cf : 1 horned 9 : 3 hornless 9 . 

Bateson and Punnet assume that the horned breeds are of the constitu- 
tion HHXX, female, and HHXY, male; and the hornless, hhXX, female, 
and hhXY, male. The i^i then consists of HhXX, hornless females, and 
HhXY, horned males. Here they make the assumption that one dose 
of the factor H results in the production of horns in the male on account 
of the sex relations, but in the female two doses are necessary for the 
development of horns. This hypothesis does not account for those 
breeds which are horned in the male and hornless in the female, but it 
is not absolutely necessary that it should. We see here, however, a 
basis for the modifications of horns following castration for evidently 
the factor complex for the horned condition reacts differently in male and 
female. 



550 GENETICS IN RELATION TO AGRICULTURE 

In birds some striking results of castration have been obtained. 
Goodale especially has made extensive investigations with ducks and 
fowls. The results of ovariotomy in Rouen ducks are shown in Plate IV. 
Early removal of the ovary results in almost complete assumption by 
the female of the strikingly different plumage patterns of the male, and 
along with them of other secondary sexual characters peculiar to males. 
If removal be later or incomplete, the assumption of male secondary 
characters is correspondingly less complete. Castration of the drake on 
the other hand has very little effect on the secondary sexual characters. 
Here if any change in characters occurs, it is toward the infantile condition 
rather than toward that of the opposite sex. Observations on Brown 
Leghorn fowls confirm these conclusions. Thus an early ovariotomized 
female developed almost a complete set of male secondary characters. 
Capons, on the other hand, exhibited almost the entire set of male sec- 
ondary characters. The characters of such birds, which are responsible 
for the current belief that capons are feminized cocks, were shown to be 
infantile characters rather than characters of the female sex. 

Finally, mention must be made of Steinach's experiments on guinea- 
pigs and rats. Steinach first castrated male guinea-pigs and rats and 
then transplanted ovaries into them. The animals thus operated upon 
became strongly feminized. Feminized rats took on the texture of hair, 
the size of skeleton, and sexual behavior of females. In both cases 
the mammary glands became greatly enlarged. Throughout the total 
changes wrought by the establishment of ovaries in the castrated male 
rats and guinea-pigs were such as to throw the whole set of developmental 
processes toward the female side. We await with interest further ex- 
periments of this kind. 

We can only conclude from these experiments that the sex glands 
actually furnish something, in the way of internal secretions perhaps, 
which affect the internal conditions under which the cells react. The 
presence of these hormones is the exciting condition for development 
of secondary sexual characters, not any fundamental factor difference 
in the two sexes, save that of the sex-determining factor itself. This, as 
Lillie has shown, is the most reasonable explanation of barrenness in 
free-martins. In mammals the effect of castration on the male is to throw 
the secondary sexual characters toward the female side, but not very 
strongly. The effect is only complete when ovaries are present. The 
female, however, is little affected. In birds the relations are reversed, 
castration of the female leads to the development of the secondary 
sexual characters of the male; castration of the male to little change. 






B 

o 
O 



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CHAPTER XXXV 
FERTILITY IN ANIMALS 

Among domestic animals fertility is of direct economic importance. 
Problems associated with it have been investigated from many different 
angles, even from the standpoint of inheritance. Unfortunately, how- 
ever, with respect to this latter feature of the question, not many inves- 
tigations have been carried out with higher animals. It is necessary, 
therefore, to seek for the principles disclosed by investigations with the 
lower forms of life, and to determine to what extent they may be applied 
to higher animals. 

Factors Influencing Fertility. — The factors which affect fertility are 
extremely numerous and varied. In considering the problems of in- 
heritance connected with it, it is, therefore, necessary to make the inevi- 
table scientific distinctions as to kinds of influences which may affect it 
and as to the different meanings which the term itself may have. In 
common parlance the term fertihty signifies ability to produce active, 
living young. In higher animals in general fertility is measured by 
the reproductive capacity of pairs of individuals. Fecundity is the 
term used to designate the potential reproductive capacity of indi- 
viduals. It is measured by the ability of the individual to form mature 
ova or spermatozoa. Fecundity can be measured accurately and di- 
rectly only in special cases such as in birds; in mammals only fertihty 
can be determined. 

Several physiological factors must be considered in a treatment of 
fertility in animals. Of these only a few can be mentioned here. For 
a more extended treatment, the student should consult treatises on the 
physiology of reproduction, of which that of Marshall is especially 
valuable. 

Among influences which lead to sterihty or decrease in fertility are 
those of domestication. Here the effect depends largely upon the idio- 
syncracies of the particular wild species which has been domesticated. 
It has been suggested that food might in some cases be the determining 
factor, the supposition being that the animal under captivity may not 
obtain the variety and character of food necessary to maintain a healthy 
condition of the reproductive tracts. By no means, however, is the 
sterihty of wild animals in captivity visibly correlated with changes 
in their mode of hfe, for often the most surprising variations occur in 

551 



552 GENETICS IN RELATION TO AGRICULTURE 

closely related species. Although it is impossible, therefore, to general- 
ize as to what particular factor of the environment is responsible for the 
condition of lowered fertility among wild animals in captivity, there 
can be no question as to the strikingly adverse effect of confinement in 
certain cases. 

Unfavorable conditions of the accessory reproductive organs occasion- 
ally cause sterility. Thus in cattle barrenness is sometimes the result 
of an acid condition of the secretions of the vagina. Simply injecting 
a weakly alkaline solution into the vagina has been found effectively 
to overcome this difficulty. The practice of artificial insemination has, 
also, been used in cases where the mucous secretions are unfavorable 
for conception, and in cases where structural bars to conception exist 
in the accessory reproductive organs. Among cattle especially conta- 
gious abortion is a serious cause of barrenness. This disease is bacterial 
in etiology, transmissible from animal to animal, perhaps usually by 
the agency of the herd bull, although possibly at times through food, 
aqd experimentally by intravenous injection. Not only does the disease 
cause abortion in animals in which it has not developed until after con- 
ception, but in animals previously infected it leads to barrenness. The 
disease is characteristic in its lesions and effects and may be controlled 
by the adoption of proper antiseptic measures. 

In general domestic animals are much more prolific than their wild 
progenitors. Several reasons for this fact may be pointed out. Those 
species which can adapt themselves to conditions of domestication 
usually find such surroundings more favorable to development and to 
the production of offspring. Moreover, there is a natural tendency for 
selection to favor the survival of those strains or races which reproduce 
most rapidly, and man has augmented this tendency by choosing the 
more prolific members of the race for breeding stock. 

But even the long-continued selective processes of domestication have 
not sufficed to attain to the maximum of fertility for the species. Few 
realize how great is the field for improvement in this respect. In England 
horse breeding, according to Marshall, suffers an enormous loss each 
year because of the failure of no less than 40 per cent, of mares selected 
for breeding purposes to produce offspring. Cattle, sheep, and swine 
appear to suffer somewhat less in this respect but the loss is far from 
inconsiderable. Heape estimates the average loss among cattle to 
amount to over 15 per cent. Among sheep the loss from actual sterility 
alone amounts to nearly 5 per cent. In view of such statistics the in- 
crease of fertility in domestic animals becomes a problem of prime 
economic importance. 

The Darwinian Theory of Fertility. — The results of Darwin's extensive 
investigations of problems of vigor and fertility in plants and animals 



FERTILITY IN ANIMALS 553 

may be summed up in the trite statement, Nature abhors inbreeding. 
From his extensive investigations Darwin concluded that all organic 
beings benefit from an occasional cross and that the inevitable effect of 
continued inbreeding is loss of size and decreased constitutional vigor 
and fertility, and at times unusual tendency toward the production of 
malformations. Since Darwin's evidence was drawn hirgely from do- 
mesticated animals, and since other serious detrim(>ntal features of in- 
breeding are pointed out in addition to loss of fertility, it is important 
that enquiry be made into the reasons why inbreeding should result in 
decreased fertility. It is, also, important to note that we are attempting 
to harmonize in this treatment Darwin's conclusions with a theory of 
heredity unknown to him. 

Inbreeding not in Itself Harmful. — Although supposed evidence of 
harmful effects of inbreeding has been presented by a number of inves- 
tigators, there is nothing in this evidence which necessarily throws the 
blame upon inbreeding in itself. A single contrary case is all that is 
necessary for establishing the negative interpretation, and there are a 
number of such cases. Thus investigations on the effects of inbreeding 
in the fruitfly have been carried out on a much more extensive scale than 
would ever be possible with any of the higher domestic animals. For 
example, Castle and his associates inbred the fruit fly for fifty-nine 
generations, mating brother with sister throughout the investigations. 
They reached the general conclusion that inbreeding unaccompanied 
by selection generally results in decreased productiveness, but that proper 
selection for high productiveness results in maintaining the original 
fertility of the race. They found further that low productiveness is 
sometimes inherited like a Mendelian recessive, as shown by its appear- 
ance in alternate generations, and that in crosses between strains of 
high and low productiveness there was evidence of segregation in F2. 

Castle further comments upon a polydactylous race of guinea-pigs 
which was descended from a single individual. They have been inbred 
for over 10 years, yet despite this fact they show no signs of diminished 
fertility; on the contrary, they are superior in size and in constitutional 
vigor to most races. Moenkhaus' results with Drosophila also seem 
to indicate that a high degree of fertility may be maintained in successive 
generations of inbreeding if sufficient care be taken to select from the 
most fertile individuals. Hyde, on the other hand, found a decrease in 
fertility consequent on continued inbreeding. The experimental results, 
therefore, show that sometimes inbreeding does not result in diminished 
fertility. The fact, however, that there are so few cases in which in- 
breeding has not been followed by measurably harmful results calls for 
some explanation. In the rest of this chapter some reasons for this 
fact will be pointed out. 



554 GENETICS IN RELATION TO AGRICULTURE 

Fertility as Related to Mendelian Factors. — There is a considerable 
body of evidence to show that some Mendelian factors exhibit residual 
effects upon the fertility of individuals which bear them. This is perhaps 
most clearly established for certain factors in Drosophila. Thus among 
sex-linked factors Morgan has shown that those for the rudi- 
mentary and the fused wing conditions are practically always associated 
with sterility. In rudimentary flies the males are fully fertile, but the 
females are usually completely sterile. Examinations of the ovaries of 
rudimentary females demonstrate that the eggs do not develop normally, 
but for the most part remain in a low stage of development. Similarly 
the mutant fused is absolutely sterile in the female sex, but fertile in the 
male. Stock must, therefore, be maintained by mating heterozygous 
females to fused males. Here again examination of the ovaries has shown 
reduction in the number of mature eggs normal for the wild type. 

Between this relatively complete sterility and the normal fertility of 
the wild type, there exist all possible gradations. In fact even in wild 
type flies as Castle and his associates and others have abundantly shown 
strains possessing different degrees of fertility exist. But mutant strains 
often exhibit lessened vigor and fertility specifically attributable to the 
residual effects of the mutant factors themselves. This effect appears 
to be cumulative, for the presence of several mutant factors often greatly 
accentuates it. The difficulty has often proven a very great obstacle in 
carrying out some Drosophila experiments, but it serves to demonstrate 
that sterility may be a consequence of certain combinations of factors. 

Specifically a number of definite cases may be given. Muller at- 
tempted to unite the factors for yellow body color, white eyes, abnormal 
abdomen, bifid wings, vermilion eyes, miniature wings, sable body color, 
rudimentary wings, and forked spines in one strain of flies. Here, of 
course, the factor for rudimentary wings in itself might be expected to 
have a profound effect upon the fertility of the strain, but aside from 
this effect it was found that the strain was so deficient in viability and 
general vigor that it was necessary to propagate it by specially devised 
breeding methods in the heterozygous condition. The heterozygous 
flies showed only an insignificant reduction in viability and fertility, 
whereas their full brothers and sisters which were homozygous for the 
recessive factors were so weak as to be of no value in the experiments. 
The same difficulties were met with in dealing with combinations of 
recessive factors belonging to other groups. It is safe to say that almost 
any combination of several recessive factors in Drosophila results in 
diminished vigor and consequent decrease in fertility. The effect is, 
however, specific, for the degree of diminution depends not only upon 
the number of recessive factors which are combined, but also upon the 
specific effects of the factors themselves. The specific residual effects 



FERTILITY IN ANIMALS 



555 



of certain Mendelian factors upon fertility cannot, therefore, well be 
denied. (See Fig. 216.) 

The Chromosomes and Fertility. — Bridges has demonstrated for 
Drosophila that males of the chromosome constitution XO, instead of 
the normal XY, are totally sterile. Here a specific chromosome differ- 
ence, the absence of the F-chromosome from the hereditary 
mechanism, leads definitely to complete sterility. Not many other 
cases are known among animals of sterility dependent upon abnormal 
chromosome constitution, but Bridges reports several known cases of 




Fig. 216. — Drosophila mutation which exhibits a high degree of sterility. 
b and c, fused wings. {After Morgan and Bridges.) 



a, Normal wing; 



aberrant hereditary behavior which may be dependent upon irregular 
chromosome distribution and content. 

Sterility in Other Animals. — In some other animals there are cases of 
sterility which suggest strongly the effect of definite Mendelian factors. 
Thus several writers have commented upon the sterility of tortoise- 
shell male cats, and apparently orange males are also, sometimes at 
least, sterile. The reason for this particular case has not yet been 
established definitely by breeding tests, and there is apparently some pos- 
sibility that irregular chromosome distribution may account for it. 

An instance from practical breeding history which appears to belong 
to this category is that of barrenness in Bates's famous Duchess family 
of Shorthorns. This family was noted for superior individual excellence, 
consequently breeders, naturally desirous of maintaining this excellence, 
followed a practice of close breeding within the family, an example of 



656 



GENETICS IN RELATION TO AGRICULTURE 



which is given in Fig. 217. But the family was tainted from the be- 
ginning with the curse of barrenness, which such a system of breeding 
must inevitably preserve. Shortsighted breeders at the time considered 
it a fortunate circumstance that Duchess cows were so often barren, 
for it kept down the number of individuals of this favorite strain and 
resulted in prices correspondingly high. But as a result of barrenness, 
the strain eventually ran out completely. In Fig. 218, an attempt 
has been made to show diagrammatically how barrenness was inherited 
in this family. The diagram is not complete, for it includes only the 
females in the family. Nevertheless it brings out very forcibly how 



Duchess 55th • 



4th Duke of Northumber- < 
land (3649) 



Duchess 38th 



Short Tail (2621) 



Duchess 34 th 



Norfolk (2377) 



Duchess 33rd 



f Belvedere (1706) 
[ Duchess 32nd 
f Belvedere (1706) 
[ Duchess 29th 

[ 2nd Hubback (1423) 

\ 

[ Nonpareil 

[Belvedere (1703) 

Duchess 19th 



Fig. 217. — Pedigree of one of the latest Duchess cows, illustrating system of close-breeding 
followed in maintaining the family. Duchess 55th produced two calves. 



barrenness occurred very early in the family history, and how it re- 
appeared in about the same proportion of the total population through- 
out its history. Far from showing an intensification of the defect as a 
result of inbreeding, this diagram merely illustrates the heredity of a 
defective family trait. 

Sterility of Hybrids. — There is a definite type of sterility which is 
referable to the effects of species hybridity. We have already had 
occasion to comment upon this type of sterility in connection with other 
matters, here we shall however refer to it again with particular emphasis 
upon certain of its aspects. 

For the higher animals we do not possess much in the way of definite 
data respective to hybrid sterility. The mule, a familiar and oft-cited 
example, appears from all accounts to be very nearly completely sterile. 
The accounts of fertility in mare mules are for the most part shadowed 
in doubt, but the possibility of a slight fertility should not be denied. 
The hinny, the homolog of the mule, exhibits as high a degree of ster- 



FERTILITY IN ANIMALS 



557 



ility as the mule. Of other hybrids within tlie genus Equus the evidence 
is even less satisfactory. Apparently the cross between the horse and 
zebra is sterile, like the mule. But the zebra and ass appear to be more 
closely related, and the possibilities of securing offspring from such 



i 



? 



*6i' 



i 



m 




^HM 



^ 



o q^ 



<b 



qiooio 




Fig. 218. — Illustrating inheritance of barrenness through the female line of the Duchess 
family of shorthorns; barren cows represented by solid black circles. 

hybrids appears to be somewhat greater. There is in fact one reference 
in the literature to a fertile male hybrid between the zebra and the ass. 

In the genus Bos, taken in the wider sense to include the subgenera 
Bison and Bibos, there are various degrees of sterility consequent upon 
hybridization. The domestic cow, Bos tauriis, gives fertile male and 



558 GENETICS IN RELATION TO AGRICULTURE 

female hybrids with the zebu, Bos indicus. With the yak, Bihos grun- 
iens; the gayal, Bihos frontalis, the gaiir, Bihos gaurus, and the bison, 
Bison qmericanus, the female hybrids with the domestic cow are fertile, 
but the males are sterile. The banteng, Bihos sondaicus, and the zebu 
behave like this latter series in giving fertile female and sterile male 
offspring. In this respect they resemble Detlefsen's and Castle and 
Wright's results with species crosses among guinea-pigs, the female 
hybrids of which were fertile, the males sterile. 

Among domesticated birds in particular the reproductive powers are 
strongly disturbed by hybridization. Not only are such hybrids often 



/?>. 






V f- 




-v\ 






Fig. 219. — Abnormal reduction divisions in spermatogenesis of the mule. {After 

Wodsedalek.) 

sterile, but very frequently the sexual organs develop in an abnormal 
fashion strongly suggestive of intersexualism of the kind exhibited by 
Goldschmidt's Lymantria hybrids. Smith and Thomas have examined 
sterile hybrids between species of pheasants. They found that very 
often ovarian degeneration or imperfect development occurs in the 
females, as a consequence of which a marked tendency exists to assume 
plumage patterns and characters peculiar to the male. 

Here we are dealing rather definitely with a type of sterility different 
from that which characterizes different families within a species or 
breed or different mutant types of Drosophila, the sterility here appears 
to be more deep-seated and strangely enough, far from being associated 
with a general diminution in vigor, the vigor and size of the hybrids are 
often very augmented. We are not surprised, therefore, to find that 
profound disturbances in the hereditary mechanism occur in such hybrids. 
Wodsedalek has shown that irregular reduction divisions occur in the 
mule (Fig. 219). Smith and Thomas have shown specifically that in 
sterile hybrid pheasants of both sexes the abnormal behavior and de- 



FERTILITY IN ANIMALS 559 

generation of germ cells begins in synapsis. They conclude, therefore, 
that sterility in pheasant hybrids depends upon the inability of homolo- 
gous chromosomes derived from different species to conjugate normally. 
In the mule, which apparently receives a different number of chromo- 
somes from each parent, a morphological cause for such a difficulty 
obviously exists, but fundamentally the difficulty must be physiological, 
for it exists as strikingly in hybrids between species having the same 
numbers of chromosomes as in the rarer cases where the species have 
different chromosome numbers. We have already discussed this prob- 
lem at length. 

Fertility as Related to Heterozygosis — In another place we have 
discussed the hypothesis that heterozygosis in and of itself has a favorable 
effect upon vigor and fertility. This hypothesis is difficult to prove or 
to disprove. With the facts, however, there can be no question. Cross- 
breeding definitely does in specific cases lead to an increase in vigor and 
fertility, a fact which has long been known. But it appears more prob- 
able, as Jones has shown, that this increase is due to the establishment 
of a more excellent factor-complex than to any mysterious stimulation 
effect of the heterozygous condition. At the same time the possibility 
of an enlarged expression of the heterozygous condition of a given pair 
of allelomorphs must not be denied, but like other effects of heterozy- 
gosis, it is probably a condition depending upon the specific nature of 
the factors concerned. As a generalization, however, it must be taken 
as not proven; certainly the work with Drosophila, which is based upon 
more definite knowledge of the Mendehan factors than any other investi- 
gations to which we can refer, does not provide evidence in support of it. 
The solution of the problem has in it much of practical importance, for 
upon the hypothesis of heterozygosis it should be impossible to build up 
a breed which would reproduce in full the complete set of excellent char- 
acters of the cross-bred. If, however, a more favorable combination of 
factors is responsible for the excellence of cross-bred animals, then it 
should be possible by careful breeding to fix them in a new breed. 

Fecundity in Fowls. — It is a genuine pleasure in a mass of contradic- 
tory and illy digested data to meet with something which gives hope for 
the same definiteness with regard to the problem of the inheritance of 
fecundity that has been attained in the anal3'^sis of the inheritance of 
other more clearly defined characters. We cannot, therefore, but com- 
mend the patient investigation and brilUant analysis to which Pearl has 
subjected the problem of the inheritance of fecundity in the domestic 
fowl. Many criticisms have been launched against his conclusions, it 
is true, but it is highly probable that these criticisms involve a funda- 
mental misconception of the nature and results of scientific knowledge. 

Pearl's results deal particularly with winter egg production in the 



560 



GENETICS IN RELATION TO AGRICULTURE 



domestic fowl. The conclusions are based upon an analysis of data 
obtained by trapnesting strains of pure-bred Barred Plymouth Rocks 
and Cornish Indian Games, and Fi individuals and F2 individuals ob- 
tained by mating Fi individuals inter se and by mating them back to 
their parents in all possible combinations. Over a thousand birds were 
subjected to this definite experimental test. 

With respect to winter egg production hens naturally appear to fall 
into three well-defined classes; (a) those birds which lay no eggs during 
the winter period; (6) those which lay something less than about thirty 
eggs; and finally (c) those which lay more than thirty eggs. Since egg 
laying is a character strongly influenced by environmental conditions 



120 



100 



80 



'60 



S40 



20 





















/ 




















/ 


















A 


/ 














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_ n- 


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90 80 70 CO 50 10 30 20 10 

Per Cent Producing Indicated Number of Eggs or More 

Fig. 220. — Contrasted flock curves of winter egg production of Barred Plymouth Rock 
(solid line) and Cornish Indian Game (broken line) pullets. {After Pearl.) 

and somatic fluctuations, these classes are not absolute, nor on the other 
hand are they by any means purely arbitrary as has been determined 
by statistical studies of flock production during the winter period. The 
differences which exist between the two breeds under investigation are 
shown graphically in Fig. 220. Taking a production of thirty eggs or 
more as the standard of comparison between the two breeds, it may be 
seen from this graph that only about 6 per cent, of the total flock of 
Cornish Indian Games produced as many eggs as this during the winter 
period, whereas 54 per cent, of Plymouth Rock pullets measured up to 
this standard of excellence. 

From a consideration of the data obtained from a wide series of 
crosses, Pearl proposes the following analysis of the inheritance of 
fecundity in fowls as measured by winter egg production. For the sake 
of clearness and conformity to treatment in the remainder of the text, we 
have used symbols different from those used by Pearl without, however, 
in any way modifying the essential features of his analysis. 



FERTILITY IN ANIMALS 



561 



Table LXXIV. — Genetic CoNSTiTunoNS of 
Barred Plymouth Rock Males for 
Fecundity Factors 



(a) The Sex Factors. — In the fowl the type of sex-inheritance is that 
designated WZ in Morgan's terminology. Females are heterozygous 
for the sex- factor; i.e., they are WZ in constitution, whereas males are 
homozygous, ZZ. W like Y in the XY type is neutral and carries no 
demonstrable factors. 

(b) A fecundity factor, L, which determines a winter egg production of 
something less than thirty eggs. It is dominant over the allelomorph 
/, which is present in fowls 

which produce no eggs 
during the winter season, 
but the homozygous con- 
dition LL does not con- 
dition a higher winter egg 
production than does the 
heterozygous condition LI. 

(c) A Sex-linked Fe- 
cundity Factor, M. — Like 
L it determines alone a 
winter egg production of 
something less than thirty 
eggs. With L, however, 
it gives a winter egg pro- 
duction of over thirty. 

Pearl was able to classify the individuals which he used in his ex- 
periments into classes according to their zygotic constitutions. For the 
sake of clearness these classes are given in Tables LXXIV, LXXV, 



Class 


Genetic 
constitutions 


Gametes produced 


1 


{ZM){ZM)LL 


{ZM)L 


2 


{ZM){Zm)LL 


{ZM)L, {Zm)L 


3 


{ZM){ZM)Ll 


{ZM)L, {ZM)l 


4 


(ZM) {Zm)Ll 


{ZM)L, {ZM)l, {Zm)L, {Zm)l 


5 


{ZM){ZM)ll 


{ZM)l 


6 


{ZM){Zm)U 


{ZM)l, {Zm)l 


7 


{Zm) {Zm)LL 


{Zm)L 


8 


{Zm) {Zm)Ll 


{Zm)L, {Zm)l 


9 


{Zm){Zm)U 


{Zm)l 



Table LXXV — Genetic Constitution of Barred Plymouth Rock Females for 

Fecundity Factors 



Class 


Genetic constitutions Z gametes 


W gametes 


(Winter egg production 


1 

2 
3 
4 
5 
6 


{ZM)WLl 
{ZM)WLL 
{Zm)WLl 
{Zm)WLL 

{Zm) Wll 
{ZM)Wll 


{ZM)L, {ZM)l 

{ZM)L 

{Zm)L, {Zm)l 

{Zm)L 

{Zm)l 

{Zm)l 


WL, Wl 

WL 

WL, Wl 

WL 

Wl 

Wl 


Over 30 eggs 
Over 30 eggs 
Under 30 eggs 
Under 30 eggs 
No eggs 
Under 30 eggs 



LXXVI, andLXXVII. These tables represent genetic constitutions which 
were realized and recognized during the course of the experiments. It 
will be observed that among the Barred Plymouth Rocks every possible 
genetic constitution was represented, but among the Cornish Indian 
Games the factor M was not contained in any individual. 

36 



562 



GENETICS IN RELATION TO AGRICULTURE 



It is difficult to present briefly all the evidence which has led Pearl 
to advance the foregoing analysis for the data on fecundity in fowls, 
reference must be made to the complete pubhshed results dealing with 
these experiments. In Table LXXVIII the data are summarized and com- 
pared with expectations, 
Table LXXVI. — Genetic Constitutions of 
Cornish Indian Game Males for 
Fecundity Factors 



Class 



Genetic constitutions 



Gametes produced 



but not in a satisfactory 
manner, because different 
types of matings have been 
lumped together. It does, 
however, show as well as 
can be shown in so short 
a summary, how closely 
fecundity conforms to the 
requirements of a Men- 
delian analysis. There can be little doubt that Pearl has laid the 
broad foundations for a more definite knowledge of the behavior in 
heredity of a complex character of great economic importance. The 
practical implications of this analysis are discussed in Chapter XXXVIII. 



{Zm){Zm)LL 
(Zm) {Zm)Ll 
{Zm){Zm)ll 



iZm)L 

(Zm)L, {Zm)l 
{Zm)l 



Table LXXVII. — Genetic Constitutions of Cornish Indian Game Females for 

Fecundity Factors 



Class 



Genetic constitutions 



Z gametes 



W gametes 



Winter egg production 



iZm)WLL 
{Zm)WLl 
{Zm) Wll 



{Zm)L 

{Zm)L, {Zm)l 
{Zm)l 



WL 

WL, Wl 
Wl 



Under 30 eggs 
Under 30 eggs 
No eggs 



Table LXXVIII. — Observed and Expected Distributions of Winter Egg Pro- 
duction for all Matings in Pearl's Experiments 




Class 


Winter production of daughters 


Mating 


Over 30 


Under 30 


None 


Barred Plymouth Rock 


Observed 
Expected 


365.5 
381.45 


259.5 
257.25 


31.0 
17.3 


Cornish Indian Game 


Observed 
Expected 


2.0 
0.0 


23.0 
25.0 


15.0 
15.0 


AWFi 


Observed 
Expected 


36.0 
26.5 


79.0 

88.75 


8.0 
9.75 


All Fi and back-crosses 


Observed 
Expected 


57.5 
68.6 


98.5 
95.0 


23.0 
15.4 



FERTILITY IN ANIMALS 563 

Conclusion. — We may conclude, therefore, fairly, that fertility in 
animals is a complex character highly modifiable under different con- 
ditions of environment, but nevertheless possessing a definite, although 
intricate and obscure, factorial basis. The character bears the same 
relation to systems of breeding as do any other characters, that is, the 
system of breeding in itself has no effect upon it, it is merely a mode of 
achieving certain results. This is clearly estabUshed by Pearl's experi- 
ments on fecundity in fowls, for they demonstrate that factors for high 
fecundity exist, and that these factors behave like other Mendelian 
factors. 



CHAPTER XXXVI 
SOME BELIEFS OF PRACTICAL BREEDERS 

It is proposed in this chapter to discuss not only some matters which 
belong to the discarded remnants of scientific thought, but also some 
beliefs of animal breeders which have not yet been subjected to the 
rigid scrutiny of scientific investigation necessary for analyzing them 
completely. 

Telegony. — By telegony is designated the supposed influence which 
a sire exerts upon the females with which he is mated such that the 
products of subsequent matings with other sires show some influence of 
the previous ones. The same phenomenon is known in popular speech as 
infection of the germ, the influence of previous impregnation, etc. 

One does not need to go far to find support for the hold which the 
belief in telegony has upon the popular mind. In certain cases the belief 
has been so strong as to affect rules of registration of pure-bred animals. 
Riley has collected from flock book records a few typical rules of regis- 
tration which are founded upon a belief in telegony. They are given 
below : 

Vermont Merino Sheep Breeders' Association. 

Eule 24. — The record of registered ewes will be forfeited if bred to rams other than 
pure descendants of importations direct from Spain, and it shall be the duty of 
members to report to the Secretary the label marks and numbers within the year in 
which they are so bred, who shall enter them on the records of the flocks in which they 
are recorded. Any member who shall fail to report according to this rule or offer 
lambs from such ewes for record shall be suspended or expelled. 
New York State American Merino Sheep Breeders' Association. 

Rule 17. — That this association exclude from its records all breeding ewes that 
have been previously bred to coarse wooled rams. 
American Rambouillet Sheep Breeders' Association. 

Rule 4. — No product of a Rambouillet ewe shall be eligible for registry after 
such ewe shall have been bred to any other ram but a registered Rambouillet. 
Dorset Horn Sheep Breeders' Association. 

Rule 6. — No ewe or ewe lambs shall be eligible for entry that have been served 
with any ram other than a pure bred Dorset Horn from the date thereof. 
Michigan Merino Sheep Breeders' Association. 

Rule 12. — The product of a registered ewe which shall at any time have been 
bred to a ram not a registered American Merino, or one eligible to register shall be 
excluded from registry. 

664 



SOME BELIEFS OF PRACTICAL BREEDERS 565 

It is of course possible that motives otlior tliari belief in telegony 
have had some influence in shaping these rules, but presumably this 
belief has been the chief reason for adopting them. At the same time 
it must be acknowledged that the popular belief in telegony is by no means 
universal. Thus E. Davenport calls attention to the fact that breeders of 
dogs are generally credited with a strong bcli(>f in telegony. Nevertheless 
a correspondence which he carried on with dog fanciers failed to disclose 
more than one case among thirty-seven which affirmed belief in telegony, 
and twenty-eight of these breeders were positively opposed to it. Since 
some credence is still given to telegony in popular circles, even if not 
among scientific investigators, a detailed account of the evidence against 
it will be presented below. 

Lord Morton's Quagga Hybrids.- — We can do no better in beginning a 
discussion of telegony than to refer to the classic example of it. Lord 
Morton's mare, for this case was accepted at its face value by no less 
an authority than Darwin. 

The details of this experiment are about as follows. Lord Morton 
bred a seven-eighths chestnut Arabian mare which had never been bred 
before to a male quagga. The result of the union was a female hybrid 
which plainly exhibited both in color and in form distinct evidence of its 
hybrid origin. The mare subsequently passed into the hands of Sir 
Gore Ouseley who bred her to a very fine black Arabian stallion. To the 
service of this stallion she bore first a filly foal and in the next year a 
colt foal. Lord Morton later examined these two colts and as a result 
of his inspection he wrote as follows to the president of the Roj^al Society: 

The 2-year-old filly and 3^earling colt have the character of the Arabian breed 
as decidedly as can be expected where fifteen-sixteenths of the l)lood are Arabian; 
they are fine specimens of that breed, but both in the color and in the hair of their 
manes they have a striking resemblance to the quagga. Their color is very marked, 
more or less like the quagga in a darker tint. Both are distinguished by the dark 
line along the ridge of the back, the dark stripes across the forehand, and the dark 
bars across the back part of the legs. The dark stripes across the forehand of the colt are 
confined to the withers and to the part of the neck next to them. Those on the filly cover 
nearly the whole of the neck and the back as far as the flanks. The color of her coat on 
the neck adjoining to the mane is pale and approaching to dun, rendering the stripes 
more conspicuous than those on the colt. The same pale tint appears in a less degree 
on the rump, and in this circumstance of the dun tint also she resembles the quagga. 

Both their manes are black; that of the filly is short, stiff, and stands upright, and 
Sir Gore Ouseley's stud groom alleged that it never was otherwise. That of the 
colt is long, but so stiff as to arch upward and to hang clear of the sides of the neck, 
in which circumstance it resembles that of the hybrid. This is the more remarkable, 
as the manes of the Arabian breed hang lank, and closer to the neck than those of 
most others. The bars across the legs, both of the hybrid and of the colt and filly, 
are more strongly defined and darker than those on the legs of the quagga, which are 
very slightly marked; and though the hybrid has several quagga marks, which the 
colt and filly have not, yet the most striking — namely, the stripes on the forehand are 
fewer and less apparent than those on the colt and filly. 



566 



GENETICS IN RELATION TO AGRICULTURE 



The strength of the evidence in this case can be understood better by 
reference to Fig. 221, which shows the male quagga which Lord Morton 
used in his experiments, the hybrid which was produced by the chestnut 
Arabian mare when bred to this quagga, and the filly which she produced 
subsequently to the service of a purebred black Arabian stallion. 




Fig. 221. — Lord Morton's male quagga, a hybrid between a Chestnut Arabian mare 
and this quagga, and a filly produced subsequently by the same mare when mated to a 
pure-bred black Arabian stallion. (After Ewarl.) 

But although many scientists have granted the weight of this evidence, 
later scientific thought has questioned strongly even the possibihty of 
such effect of the male upon the female. Accordingly many adverse 
criticisms have been made against the vahdity of this case, some of which, 
as for example that of J. Wilson, go so far as to deny the hybrid origin of 



SOME BELIEFS OF PRACTICAL BREEDERS 567 

the first foal produced by Lord Morton's chestnut Arabian mare. Those 
criticisms suggested by Ewart, however, because they are tempered by 
abundant experimental research, are, perhaps, most just. 

Accepting the hybrid nature of the first foal, the question arises as to 
how common striping may be in horses, especially those of Oriental 
ancestry. On this point there is abundant evidence as Ewart points 
out. The old yeliow-dun horses of the forest type, which have had much 
to do in the origin of modern breeds of horses, characteristically possessed 
a broad dorsal band and zebra-like bars on the legs, and in addition to 
these markings they often possessed faint stripes on face, neck and withers. 
In fact evidence points to the belief that a remote ancestor of this forest 
horse was probably as richly striped as some modern zebras. Even today 
it is a very common thing among mongrel ponies to meet with individuals 
which possess distinct markings suggestive of the forest horse. They are 
not uncommon among Arabian crosses. Consequently we are not sur- 
prised when this later filly is compared with the quagga to find that its 
pattern, rather than suggesting residual effect of the previous impreg- 
nation by the quagga, strongly indicates reversion to some ancestral type. 
The bars on the legs, for instance, were more marked on the hybrid, on the 
filly, and on the colt than on the quagga. The scanty mane and tail 
upon which Lord Morton dwells may simply be regarded as additional 
evidence of reversion. 

The Penycuik Experiments.- — All the debates which may center around 
Lord Morton's mare, however, do not carry a fraction of the weight in 
assigning telegony to the limbo of discarded doctrines of the experimental 
work of the last two decades. The Penycuik experiments were designed 
by Ewart to determine whether such a doctrine as telegony were tenable, 
and, if so, to what extent it exerted influence in animal breeding. They 
have been carried out on a considerable scale, and included experiments 
not only with the Equidse, but also with other animals. In every case 
Ewart was forced to the conclusion that alleged cases of infection may be 
accounted for most easily and most satisfactorily as instances of rever- 
sion to ancestral types. 

To illustrate Ewart bred the Burchell zebra stalUon Matopa to a chest- 
nut polo pony. She produced as a result of this mating twin hybrids. 
The following year she produced a foal to a light chestnut thoroughbred 
stalUon, after which she was again bred to Matopa, and produced a third 
hybrid foal. Subsequently she produced another foal to the service of a 
dark chestnut thoroughbred staUion. The three hybrid foals from this 
mating were all richly striped, in fact the stripes were more numerous, 
although less conspicuous, than those of the zebra sire. In spite of this 
fact, however, the two foals produced by mating Valda to the thorough- 
bred chestnut stallion in no particular, either in color or in form, resem- 



568 



GENETICS IN RELATION TO AGRICULTURE 



bled the hybrid foals. They were chestnut in color without any sug- 
gestion of striping, and in liveliness of temperament or vigor of develop- 
ment neither of them resembled in the least the hybrid progeny. 




Fig. 222. — Jerry, a male Grevy zebra, Equus Grevii, used in U. S. government breeding 
investigations. (After Rommel.) 




Fig. 223. — A registered Morgan mare, iiaby Gates, used ia U. >S. government breeding 
investigations. {After Rommel.) 

A subsequent experiment is of interest because of the closeness with 
which it agrees with particulars of the Lord Morton case. Ewart bred 



SOME BELIEFS OF PRACTICAL BREEDERS 



569 



Mulatto, a black West Highland pony, to Matopa and obtained a colt 
foal Romulus, a beautiful, distinctly striped hybrid. The mare Mulatto 
was then bred to a black Arabian stallion. To this service she produced 




Fig. 224. — Juno, a zebra-mare hybrid produced by mating the Morgan mare, Baby Gates, 
to the Grevy zebra, Jerry. (After Rommel.) 




Fig. 225. — Georgia, a registered Morgan filly produced by Baby Gates subsequent to 
the production of the zebra-mare hybrid Juno, to the service of the Morgan stallion, Pat 
Murphy. There is no trace of telegony. {After Rommel.) 

a foal which, when examined immediately after birth, showed numerous 
indistinct markings, so faint, however, that their exact nature was in 
some doubt. Subsequently Mulatto produced another foal to the serv- 



570 GENETICS IN RELATION TO AGRICULTURE 

ice of a dark brown West Highland stallion which was also indistinctly 
marked. 

In themselves these foals suggested as strongly that telegony might 
occasionally occur as did those described by Lord Morton. But Ewart 
tested the matter further by breeding two dark West Highland mares 
closely related to Mulatto to the same black Arabian stallion which had 
sired the striped foal. Two foals were produced, one of which possessed 
the same sort of indistinct markings as those characteristic of the foals of 
Mulatto, the other was much more distinctly striped. There can be no 
question, therefore, that the striping of Mulatto's foals was a consequence 
of normal hereditary processes having nothing to do with telegony. 

Further evidence as to the non-tenability of the doctrine of telegony 
might be cited from the Penycuik experiment. A vast amount of ad- 
ditional evidence has been obtained from other experiments, some of 
which have been performed with the distinct object of testing the 
doctrine, others for different purposes. The experiments of Baron de 
Parana with zebra hybrids closely paralleled those of Ewart and yielded 
likewise no evidence in support of the doctrine of infection. The series 
of photographs shown in Figs. 222 to 225 have been drawn from an article 
by Rommel describing the work of the U. S. government with hybrids 
between different species of Equus. Here again there is no evidence 
that Georgia the Morgan filly which Baby Gates produced subsequently 
to the production of the zebra hybrid Juno shows any effect of the pre- 
vious impregnation. The apparent stripes on the body of Georgia, it may 
be mentioned in passing, are merely her ribs showing through. Similarly 
there is no evidence that Sweepstakes, the dam of Star Pointer and other 
pacers, was in any way influenced by the fact that previously she had 
borne two mule foals. The evidence from mule-breeding establishments, 
in which thousands of mules have been produced, in every dependable 
instance is against the doctrine of telegony. Mumford has recorded 
a large number of concrete cases in support of this position. In a few 
instances mares had produced as high as ten or eleven mule foals before 
they were bred to stallions, yet in not a single case was there positive 
evidence of telegony. 

The development of the Mendelian theory of heredity has robbed 
most of the old evidence for telegony of all its value. An instance which 
Ewart quotes is of considerable interest in this connection. A tan Dachs- 
hund bitch was bred to a tan dog, and produced a litter of puppies 
having pure white bodies and tan cheeks and ears. Now this bitch had 
previously borne by misalliance a litter of puppies to a white Fox terrier 
with tan cheeks and ears. Presumably both the tan Dachshund bitch 
and dog had long lines of tan or black and tan ancestors ; what more natu- 
ral than to conclude that this was a strict case of telegony? But the 



SOME BELIEFS OF PRACTICAL BREEDERS 571 

breeder to whom the attention of this instance was called remarked that, 
although the color of this litter was strongly reminiscent of the white fox 
terrier, the form and general characteristics were otherwise those of pure 
Dachshunds. Accordingly he traced the pedigree of the dam and found 
that in the sixth generation it ran back to the kennel of a lady whose' 
hobby was white Dachshunds with tan cheeks and ears. This particular 
mating had simply given opportunity for the expression of latent factors 
carried by the tan dogs. It was a perfectly intelligible case of rever- 
sion, not telegony at all. 

Harmful Effects of Hybridization.— Although we cannot accept the 
belief in telegony, we must admit that bearing hybrid offspring may 
sometimes have detrimental effects upon the dam. Thus Ewart quoting 
from Baron de Parana calls attention to the practice in Brazil of breeders 
of mules putting their mares to horses after they have reared two or 
three mules in order to prevent them from becoming sterile. There is 
a possibility that a hybrid fetus in consequence of its unusual vigor 
may tax more strongly the resources of the dam and in a sense impoverish 
her. This is particularly the case in some hybrids like those between the 
bison and domestic cattle, the production of which is a tremendous 
drain upon the dam's system and often leads to fatal consequences. 
But this is not telegony, it is merely a consequence of disturbing the 
physiological balance in the dam, and has nothing whatever to do with 
the transference of the characters of a previous sire to offspring borne 
subsequently to the service of another sire. 

Infection of the Male. — The belief in infection of the male is by no 
means as strong as that in telegony, but occasionally it is met with. 
Ewart recites an incident of a breeder who refused to allow his Jersey 
bull to serve Shetland cows for fear that the bull would subsequently 
carry over old Shetland traits into his Jersey herd. Since we have, 
however, discarded telegony as applied to the female, there appears to 
be no warrant whatever for considering it in the male, where an effective 
mechanism of operation is even less conceivable. Moreover, a vast 
amount of evidence which has been obtained in Mendelian experiments 
leaves no room whatever for this belief. 

Saturation. — The doctrine of saturation is fundamentally based 
upon a belief in the cumulative effect of telegony. It holds that succes- 
sive children of given parents come to resemble the sire more and more 
in their characters. Although this doctrine has been accorded some im- 
portance at times, like the doctrine of telegony it finds no support from 
experimental evidence. Here again we may point to the evidence from 
Mendelian experiments, collected for another purpose, it is true, but 
yielding direct evidence in opposition to the belief in saturation. More- 
over, Pearson has collected statistical evidence in human beings with 



572 GENETICS IN REALTION TO AGRICULTURE 

respect to the stature of successive children in the same famihes. He 
finds no evidence whatever of saturation, or as he states it of a "steady 
telegonic influence." 

Maternal Impression. — The behef in maternal impression, or in the 
effect of the pregnant mother upon her growing fetus, is one of the en- 
during tenets of popular faith. We need not trouble ourselves here with 
the long series of influences which are supposed to pass from the mother 
to the unborn child in human beings. Suffice it to state that they are no 
more varied nor yet more tenable than those cases which have been 
described for domestic animals. 

Many curious cases from that of Jacob's peeled wands^ down to those 
of far more recent times might be cited from the chronicles of maternal 
impression; but like the belief in telegony, they all spring from the un- 
scientific attitude of the popular mind toward isolated instances. We 
suspect for instance that the famous Biblical herdsman used other 
methods than that of the peeled wands in order to achieve his remarkable 
results. 

An instance may be given as typical of those which are recounted in 
support of the belief in the effect of maternal impressions, although in reality 
it is stronger than most cases. A section of a well-known Scottish herd 
of Aberdeen-Angus cattle which was separated from an Ayrshire herd 
by only a wire fence persistently produced, for several successive genera- 
tions, red and black-and-white calves. But this was the formative period 
of the breed, and we have already had occasion to mention the diversity of 
color which characterized Aberdeen-Angus foundation stock. The oc- 
currence of red and black-and-white calves, therefore, is a simple con- 
sequence of the cropping out of recessive factors, the Mendelian ex- 
planation is adequate and satisfactory. Moreover it is not entirely 
improper for us to call attention to the utter confusion which would pre- 
vail in herds of Aberdeen-Angus cattle, if this phenomenon were of gen- 
eral occurrence. We venture to state that very few breeders of solid- 
colored cattle have had such trouble from the proximity of herds of 
Holstein-Friesian, Shorthorn, Ayrshire, and other breeds of cattle, not 
to mention other sources of contamination which might occur. 

But some of the legends of Aberdeen-Angus history are even more 
curious than this one. It is recorded of the famous breeder McCombie of 
Tillyfour that he erected a high black fence around his breeding paddock. 
But it may be expected that McCombie having as his ideal the black 
polled Aberdeen-Angus cattle used other means of securing a strain 
breeding pure for the typical Aberdeen-Angus characters. 

Like the belief in telegony, the belief in maternal impressions arises 
from an unscientific attitude of mind toward evidence in general. The 

^Cf. Genesis 30:31—43. 



SOME BELIEFS OF PRACTICAL BREEDERS 573 

particular, unusual instance, because it is so striking fixes itself in the 
memory and the countless thousands of cases which do not support the 
doctrine are overlooked. There is something in it akin to the memory 
of the card player which retains so tenaciously the recollection of an 
unusual hand, but here it is usually clearly recognized that chance 
alone is responsible for the good fortune. So also in animal breeding 
remembrances of strange coincidences are longest borne in mind, but it 
seems to be a very common fault not to realize that they are after all 
nothing but coincidences. 

Prepotency. — It has been an early observation of animal breeders 
that some animals possess a superior power of impressing offspring 
with their characters. This is precisely what is meant by prepotency; 
a prepotent animal is simply one which has the power to stamp its 
offspring with its own characteristics. Obviously there is much room 
here for confusion of thought, but at least the existence of prepotent 
animals can scarcely be denied. The science of genetics unfortunately 
has not advanced far enough to be able to state precisely what are the 
requisites for prepotency, nor has it progressed to such an extent, as 
some seem to think, that prepotency, like those other popular doctrines 
which have been considered in this chapter, may be analyzed completely 
and its untenable features discarded. 

The fact of prepotency we say must be admitted, and this position 
is justified by a study of the history of any of the established breeds of 
domestic animals. Without exception such breeds all show a narrowing 
of the ancestral lines to a few favored families due to the superior excel- 
lence and transmitting power of the individuals belonging to the family. 
For prepotency is obviously a family matter. 

One of the most notable instances of prepotency is that of the Ham- 
bletonian family of trotters and pacers. The progenitor of this family 
was Hambletonian 10, a remarkable stallion who appears to have 
inherited his excellent characters from those famous imported sires of 
the early days of speed development. Messenger and Bellfounder. 
Hambletonian 10 himself was no mean performer, having to his credit 
a record of 2 :48 }i as a 3-year old in 1852, at which time the fastest 
trotting record was 2 :28; but it is as a breeder that he has won enduring 
fame. 

E. Davenport has studied with considerable care the relation of prepo- 
tency to the development of trotting and pacing horses in the United 
States. He found that up to and including 1901, a total of 26,327 horses 
had been admitted to the list of performers, i.e., had records of 2 : 30 or 
better. Of these performers, 14,808 traced back to eighty-five grandsires. 
In other words over 50 per cent, of performers traced back to slightly 
more than 1 per cent, of the grandsires of the breed. This fact is 



574 GENETICS IN RELATION TO AGRICULTURE 

truly a remarkable demonstration of the relation of prepotency to the 
development of speed in the American Standard bred. But even more 
remarkable is the record of the ten greatest producers of speed up to that 
time. They are given in Table LXIV. Of the ten great sires here listed, 
one is by Hambletonian 10, eight have Hambletonian 10 for grandsire, 
and finally Nutwood, the greatest in the list, is by Belmont, by Adallah, 
by Hambletonian 10. Every one of the ten premier stallions of the breed, 
therefore, belongs to the great Hambletonian family. 

Further evidence as to the existence of prepotency has been given 
from time to time for many other breeds and for different characters. 
A typical investigation of this kind has been conducted by Hover for 
butter-fat production of pure-bred Guernsey cattle. From the advanced 
registry records for this breed up to December, 1915, Hover found that 
only thirty-two sires had produced three or more daughters having records 
equivalent to 600 pounds of butter fat at maturity. Of these thirty- 
two sires only three had produced more than ten such daughters, and 
all of these belong to the May Rose family. This same family contains 
six more of the thirty-two superior sires. Of the rest the Masher family 
contains seven; the Governor of the Chene family, five; the Glenwood 
family, five; and the Sheet Anchor family, six. Some of the sires of 
course belonged to two or more of these families. The results are in no 
particular different from those which might be obtained with any other 
dairy breed. 

A demonstration of the existence of prepotency, however, is far from 
a scientific treatment of the subject. While many geneticists admit that 
prepotency is as yet an unsolved problem, they have not failed to point 
out several ways in which prepotency might operate. These suggestions 
have pointed to the relations of dominance and recessiveness, to variations 
in the potency of factors themselves, and to interrelations within the 
hereditary complex as providing firm bases for the existence and in- 
terpretation of prepotency. We shall discuss each of these briefly below. 

The Mendelian Interpretation, — That interpretation of prepotency 
which refers it solely to the particular characters and the relations of 
dominance and recessiveness within them may be called the Mendelian 
interpretation. The simplest expression of this interpretation is found 
in the relation of homozygous dominants to those which are heterozygous. 
These two classes are often indistinguishable phenotypically, but the 
homozygous dominant when mated to recessives impresses its characters 
on all the offspring, whereas the heterozygous dominant only impresses its 
characters on half the offspring. The practical bearing of prepotency 
of this kind may be seen by reference to Pearl's analysis of the inheritance 
of fecundity in domestic fowls. Here a Barred Plymouth Rock male 
of the genetic constitution {ZM){ZM)LL will transmit high laying 



SOME BELIEFS OF PRACTICAL BREEDERS 575 

qualities to all his female offspring regardless of the genotypes of the 
females to which he is mated, whereas one of the genetic constitution 
{Zni){Zm)ll would transmit low egg laying capacity to such an extent 
that among his daughters even from high producing hens none would 
fall in the high producing class. For favorable characters the validity 
of this interpretation depends upon dominance of the determining factors, 
a condition by no means universally fulfilled. 

The Relative Factor Potency Interpretation. — There is some evidence 
that the potency of a given factor sometimes varies with the source from 
which it is derived. Pearl has suggested for example that the factor L 
when derived from the Cornish Indian Game has a lower absolute fecun- 
dity value than that of the same factor in the Barred Plymouth Rock. 
The suggestion amounts to an application of the hypothesis of multiple 
allelomorphism, a graded series of multiple allelomorphs of differing 
potencies, or different relations with respect to dominance, being con- 
ceived to determine the absolute degree of expression of the factors. We 
recall here Detlefsen's work with the agouti factor of the wild Cavia 
rufescens which was recessive to the agouti pattern of the tame guinea- 
pig, and less decided in its phenotypic expression. The conclusions of 
Goldschmidt that races of the gypsy moth exist which have sex factors 
of various potencies, such that crosses between them give series of 
intersexual forms, while less definite with respect to the actual factors 
involved, provides some evidence in support of the belief that some of the 
phenomena of prepotency are dependent upon actual differences in the 
factors themselves. 

The Hereditary Complex Interpretation, — The characters for which 
families are prepotent are evidently often complex, as for example 
speed in horses, total butter-fat production in dairy cows, beef con- 
formation in cattle, and so on. They must, therefore, depend upon 
a favorable genetic constitution with respect to series of factors. 
This interpretation is based upon the conception that factors form 
physico-chemical reaction systems and it follows the lines which have 
been developed in the application of this hypothesis to species hybrids. 
We have pointed out for instance that varieties of Nicoiiana tabacum 
impress their total set of characters upon the hybrids with iV. sybestris 
because of the dominance of the tabacum reaction system. Certain 
characters which are recessive within the tabacum group are expressed 
in such species hybrids apparently because of their interrelations with 
other factors in the tabacum group. This idea is also borne out by 
certain of the Drosophila experiments. Thus Morgan notes that the 
factor for truncate wings, usually recessive, is dominant in races which 
have the black factor. The hypothesis rests upon a belief that sometimes 
factor interrelations determine whether a particular member of an 



576 GENETICS I^ RELATION TO AGRICULTURE 

allelomorphic pair shall be dominant or recessive ; and that this influence 
becomes stronger when large sets of factors determine a particular 
character. 

Greater Prepotency of the Male. — There has been a decided tendency 
to credit the male with greater prepotency than the female. Many 
investigators have pointed out that extra-biological influences such as 
the more rigid choice of males and the greater opportunity they have 
for impressing offspring may account for this belief among animal 
breeders. Some of the statistical evidence which Pearson has collected 
on this point seems to indicate no constant behavior in this respect. At 
the same time it should be noted that phenomena of sex-linkage and 
crossing-over may play an important role here. The operation of the 
former we see in Pearl's investigations of fecundity in fowls. Here the 
male is obviously the more prepotent with respect to the. transmission 
of fecundity. The operation of the latter we see in Drosophila experi- 
ments. Here there is no crossing-over in the male, as a consequence of 
which hybrid males more often transmit the particular set of factors 
which determine a phenotype like their own than do hybrid females. 
While the possibility of extending this phenomenon to mammals appears 
to have been destroyed by Castle's work with rats, which demonstrated 
the occurrence of crossing-over in the male, nevertheless as a possible 
factor in relative prepotency of the sexes it should not be ignored. 

Conclusions ivith respect to prepotency. For the present then we 
must regard prepotency as an established fact, a phenomenon which has 
not yet been subjected to scientific analysis. From a biological stand- 
point, however, it is clear that even with our present restricted knowledge 
there is room for prepotency based upon the existence of different kinds 
of relations between factors. 



CHAPTER XXXVII 
METHODS OF BREEDING 

Like modes of research, methods of breeding are the means by which 
certain results are attained. It is necessary to emphasize this fact, 
because even yet there is much confusion in the minds of breeders as to 
the relation which a particular method of breeding bears to results which 
have been produced by its employment. Not infrequently statements 
are made to the effect that a certain method of breeding is the cause of 
the excellence of one race or strain or the inferiority of another. There 
is a wide difference between the method of producing a given result, and 
the cause of its attainment. For the sake of clarity of thought we shall 
endeavor to emphasize this distinction, so far as is possible in the present 
state of our knowledge, in the discussions which follow. 

Phenotypic Selection. — ^The oldest method of breeding was simply 
that of mating together the most excellent individuals. In popular 
phraseology this is the method of breeding from the best — ^its funda- 
mental postulate is expressed in the old statement, like produces like. 
We have called it the method of phenotypic selection in order to empha- 
size the fact that the basis of choice for breeding in this method is the 
sum total of expressed characters of the individual. 

It is not necessary to recount here at any great length the sort of 
improvement which has been effected in modern breeds of domestic 
animals by the application of this method of breeding. Let it be sufficient 
to state that much of the excellence of modern breeds is an earnest of the 
efficiency of phenotypic selection as a mode of breed amelioration. It 
may, also, be stated justly that all later methods of breeding; out- 
breeding, line-breeding, inbreeding, and genotypic selection; are simply 
refined methods of breeding from the best — they are methods of pheno- 
typic selection plus something else; the something else usually ill-defined, 
but sometimes, as in genotypic selection, more definitely conceived. 

The limitations of the cruder form of phenotypic selection depend 
upon two primary causes, somatic modifiability of characters and geno- 
typic differences among like phenotypic individuals. Since differences 
which are due to modifiability tend in the long run to group themselves 
around a mean in the form of a normal variability curve, it may be stated 
dogmatically that long-continued phenotypic selection should tend to 
obliterate them. But it is not enough for the practical breeder to know 
37 577 



578 GENETICS TN RELATION TO AGRICULTURE 

that eventually a given result may be produced, his time is limited and 
he, therefore, desires, and rightly, to achieve a given result in the shortest 
possible time. A case in point is that which we have already discussed 
in some detail, modifiability in relation to selection for high egg produc- 
tion in the domestic fowl. Here Pearl found that modifiability was so 
great that simple phenotypic selection of the highest producers for breed- 
ing stock resulted in no improvement whatever in laying capacity. On 
the other hand, the application of a method of breeding which fully 
allowed for this effect of modifiability and which further took into account 
the germinal constitutions of the fowls selected for breeding purposes 
immediately resulted in gratifying improvement. With most characters 
the influence of modifiability is not so great as in fecundity of fowls. 
Often in fact modifiability may actually be utilized to advantage by the 
breeder in determining relative excellence. Thus any system of develop- 
ment which tends to call forth the highest possible expression of the 
capabilities of individuals tends to widen the differences between superior 
and inferior individuals. Both good and poor dairy cows tend to give 
increased milk yield when fed richly, but the increase is often more marked 
in the good cows. On the other hand, with horses in general it is possible 
by training to increase speed, but it is a question whether the increase in 
such a case is more marked in good or poor horses. We may say with 
confidence, however, that here training by developing the full capa- 
bilities of the animal tends to bring its speed up to such a standard 
that when compared with breed records, the superior excellence of the 
individual is definitely established. Modifiability, therefore, is on the 
.one hand a factor which tends to decrease the possible effectiveness of the 
method of breeding from the best; on the other hand, if properly utilized 
it is a powerful aid in the accurate selection of those individuals which 
possess the highest inborn capabilities. 

When we come to consider the influence of germinal diversity in 
phenotypic selection, we approach more nearly the problem of the real 
limitations under which the method of phenotypic selection labors. Here 
we may distinguish different ways in which germinal diversity may hinder 
phenotypic selection. 

Phenotygic Selection Does Not Distinguish Between Homozygous and 
Heterozygous Individuals. — To the student of Mendehsm this diffi- 
culty requires no further comment. It may be pointed out, however, 
that the difficulty increases as the number of factors for which selection 
is being practised increases. As with modifiability, however, this diffi- 
culty tends to be obliterated by long-continued selection, for such 
selection inevitably increases the proportion of homozygous individuals 
within a given phenotype or standard of selection. Roughly it may be 
said that the rate of increase of the proportion of homozygous individuals 



METHODS OF BREEDING 579 

is inversely proportional to the number of factors concerned in the 
selection, for the greater the number of factors the slower is the rate at 
which the population approaches a uniformly homozygous condition. 
Theoretically complete attainment of this condition is only reached after 
an infinite number of generations, but practically the number of genera- 
tions which is necessary to measure up to within 5 per cent, of the 
possible hmit is much smaller. It is, however, often so large that the 
animal breeder would prefer to use some other method, if by so doing, 
he could more quickly reach the desired standard of excellence and 
stability of type. 

At this point, however, it should be mentioned that selection is often 
made for characters which are recessive, or which give intermediates 
when in the heterozygous condition. In such cases, of course, the 
relation between phenotype and genotype is simpler and methods of 
selection gain in effectiveness in consequence thereof. 

Phenotypic Selection Does Not Make Allowance for the Differences 
Which May Exist Among the Genotypes of a Given Phenotype. — 
Simple examples of this proposition may be quoted without number. 
In fowls for example there are dominant whites like the White Leghorn 
and recessive whites like the White Plymouth Rock. The diverse progeny 
which is obtained by mating these two breeds together has been described 
in detail in a previous section. There is some evidence that a similar 
condition may obtain in cattle with respect to white coat color. White 
is, likewise, dominant in the horse, and may therefore conceal a large 
number of latent factors. In the pig the same differences in behavior 
with respect to white coat color have been noted. There is reason to 
believe that the same kind of diversity in genetic constitution obtains 
for economic characters, as for those not so strictly utilitarian. The 
breeder who follows a method of phenotypic selection should not, therefore, 
be surprised if crossing different strains results in a disappointing lack of 
uniformity in his herd. It is not difficult to see that in differences of 
genotype such as have been noted here, the breeder of best to best meets 
one of his most perplexing problems. 

Phenotypic Selection Fails to Allow for Heterozygosis. — In other 
portions of this book the assumed effect of heterozygosis on vigor and 
fertility has already been discussed at considerable length. If a hetero- 
zygous condition ever can determine a more vigorous development than 
the homozygous condition, then the breeding practice of the future will 
sometimes be materially altered in order to take advantage of this fact. 
But aside from this possible difficulty there is sometimes a very real diffi- 
culty in the fact that selection has set as its standard a type absolutely 
conditioned by a heterozygous genotype. The striking and ever-quoted 
instance of this fact is the Blue Andalusian fowl, which no amount of 



580 GENETICS IN RELATION TO AGRICULTURE 

breeding has ever been able to establish in a pure form. If more than 
one pair of factors is concerned in such a case, the progeny is correspond- 
ingly of greater variety- — it becomes a case of the Blue Andalusian fowl 
on a larger scale. It is probable that this condition is not often met 
with. The only remedy for it is to change the standard of selection. 

Pedigree Breeding. — If to phenotypic selection be added the concep- 
tion of family excellence we obtain the foundation upon which pedigree 
breeding is based. Pedigree breeding, therefore, is merely a refined 
system of phenotypic selection; in one form or another it is a very old 
system of breeding. The principle of pedigree breeding is a laudable 
one, for it judges the individual not only upon its own expressed charac- 
ters but also upon those which its ancestors have exhibited. It is, 
therefore, one more step in the direction of strict genotypic selection. 

From an ideal standpoint the effect of pedigree breeding is to empha- 
size the value of breeding ability. The existence of strikingly prepotent 
animals must have been a large factor in the development of this method. 
By insisting upon breeding ability as a measure of excellence, the tendency 
has been to eliminate the effects of modifiability and heterozygosis, and 
to favor the selection of the most excellent homozygous individuals for 
breeding purposes. By so much it has concentrated blood lines within 
breeds to a few of those which have proven most excellent, and thereby 
it has amply justified its adoption as a method of breeding practice. 

The weakness of the method lies not so much in inherent defects as 
in the uses to which livestock men have put it. The establishment of 
herdbooks in which pedigrees are recorded, while undoubtedly an impor- 
tant step in advance in the history of any breed, has tended to empha- 
size unduly the value of pedigree, often to the extent that individual 
excellence has not been rigidly insisted upon and even inherent family 
defects, hke the barrenness of the Bates' Duchess Shorthorns, have been 
regarded Ughtly. It cannot be too strongly insisted upon that the 
fundamental basis of pedigree breeding, as well as all other systems of 
breeding, is individual excellence. No matter how favorable the ances- 
try, an inferior individual within a family of superior excellence is likely 
to have lost one or more of the factors upon which that excellence is 
based. If that is the case, use of such an animal for breeding purposes 
merely increases the number of animals which lack that portion of the 
favorable genotype and by so much multiplies inferiority within the 
family and breed. There are numerous instances in breed history of 
pedigree fads which have worked to the ultimate disadvantage of excel- 
lent families because of the undue prominence given to ancestry in 
selecting breeding animals. 

Breeding Systems Based on Blood Relationship. — The influence that 
kinship has had on marriage laws in human society is familiar to all 



METHODS OF BREEDING 681 

educated people. The old Mosaic laws forbade the marriage of closely 
related individuals, and complex systems of marriage apparently directed 
against consanguineous marriages are found even among many uncivilized 
tribes of peoples. Undoubtedly the existence of these systems of mar- 
riage in human society has had some influence in shaping the methods 
which have been adopted by animal breeders, but at most the influence 
has been small. The most potent factor in livestock practice has un- 
doubtedly been the type of results which has been attained by following 
one system or another, and the general utility which the given system 
has in the hands of the average breeder. With respect to the degree of 
kinship permitted in matings there are three general systems of breeding: 
out-breeding, in which consanguinity is avoided as much as possible; 
line-breeding, which is based upon matings of moderate blood relation- 
ship; and inbreeding, which is based upon matings of animals closely akin 
to each other. Each of these methods of breeding will be discussed below. 
Inbreeding. — Specifically inbreeding is a system of breeding in which 
sire is bred to daughter, dam to son, or brother to sister. It is, therefore, 
based upon the closest possible types of mating. This system of breeding 
naturally has had its origin in the desire to intensify the blood of notably 
superior individuals. The most used form of it, perhaps, is that in which 
a famous sire is bred to his daughters and even at times to the second 
generation of daughters which have been produced by inbreeding. The 
method of inbreeding has been particularly useful in fixing types in the 
earty, formative period of the breed. It was the powerful tool which 
that great breeder of the eighteenth century, Robert Bakewell, employed 
in the improvement of horses, cattle, and sheep; and with astonishing 
success. Evidently at that time the popular prejudice existing against 
inbreeding was even stronger than it is todaj^, as we may judge from 
the statements of Culley written in 1794. 

The great obstacle to the improvemejit of domestic animals seems to have arisen 
from a common and prevailing idea amongst breeders — that no bull should be used in 
the same stock more than three years, and no tup more than two; because (say they) 
if used longer, the breed will be too near akin, and liable to disorders; some have im- 
bibed the prejudice so far as to think it irreligious; and if they were by chance in posses- 
sion of the best beast in the island, would by no means put a male and female together 
that had the same sire, or were out of the same dam. Mr. Bakewell has not had a 
cross for upward of twenty years; his best stock has been bred by the nearest affinities; 
yet they have not decreased in size, neither are they less hardy, or more liable to 
disorder; but, on the contrary, have kept in a progressive state of improvement. 

Culley might have written in the twentieth century, for even today 
inbreeding is popularly blamed for a variety of ill effects. Of these we 
have discussed decrease in fertility and vigor somewhat, and have reached 
the tentative conclusion that inbreeding of itself does not always result 
in diminished vigor and fertihty, and therefore in all probability does 



582 GENETICS IN RELATION TO AGRICULTURE 

not stand in any causal relation to it. As a method of breeding, however, 
it gives abundant opportunity for a race which has any defects what- 
soever to express them, for by simplifying the genotypic constitutions of 
the animals within a family and making them like each other, it tends to 
increase the proportion of recessive defectives produced in the family," 
But if no inherent defects exist in the family, then such an effect cannot 
be produced, and the practice is on the whole to be commended. 

The advantages of a system of inbreeding are found in the close ap- 
proach which this method makes to a strict method of genotypic selec- 
tion. It overcomes that difficulty of a system of phenotypic selection 
which arises from the possibility of mating different genotypes which 
are alike phenotypically. By this method the breeder is assured of 
genotypic identity in his breeding stock, because they have received 
their germinal elements from a common ancestor. Accordingly we are 
not surprised that this method has proven so notably successful in fixing 
types in the formative period of a breed's existence, because it is just 
at this time that both genotypic and phenotypic diversities are most 
common, and the difficulties arising from their existence most baffling. 
The increase in prepotency which is universall}^ acknowledged to ac- 
company inbreeding is in entire harmony with this interpretation — for 
by simplifying the genotypic constitutions of the individuals the tendency 
is to secure more and more individuals which are homozygous for all 
or nearly all the favorable germinal elements, and which possess in con- 
sequence of this fact superior transmitting capacity. The breeder who 
would add the practice of inbreeding to his operations must learn to 
cull with a firm hand, however, whenever defects appear for they indicate 
inevitably that some necessary constituents of the hereditary material 
have been lost. If he can do this, he has added a powerful instrument 
for improvement to his breeding equipment. 

Line -breeding. — The term line-breeding designates breeding within 
a given line of descent. By common agreement the term does not in- 
clude inbreeding; it begins with those degrees of relationship which are 
just outside the pale of inbreeding. It is, therefore, a system of breeding 
in which cousins of different degrees are mated with each other. 

No system of breeding has been so popular or so generally productive 
of good results as line-breeding. Like inbreeding, it is a method of breed- 
ing which approaches as nearly as present knowledge will permit to the 
ideal of genotypic selection. Because the individuals which are mated 
belong to the same line of descent and exhibit similar sets of characters, 
it is logically just to conclude that they possess similar sets of germinal 
elements. In this fact we have the whole explanation of the uniformity 
of progeny which is so characteristic of continued line-bi'eeding. 

Line-breeding is popularly credited with all the excellencies of in- 



METHODS OF BREEDING 583 

breeding and a greatly lessened tendency toward the production of de- 
fectives. There is a measure of truth in this belief for line-breeding, 
by the mating of animals of slightly wider relationship than those used in 
inbreeding, permits the introduction and intermingling of hereditary ele- 
ments from slightly different lines of descent. It is in this that we must 
seek the explanation for the greater success which line-breeding has had 
among most practical breeders. That explanation is not far to seek, for if 
the production of defectives depends upon factors which are distributed 
in Mendelian fashion, and there is no reason to believe that it does not, 
then any introduction of diverse hereditary elements is likely to result 
in the neutralization of the defective elements in both hereditary systems, 
for only under unusual conditions would such elements be identical 
in the two systems. Along with this tendency toward decreased pro- 
duction of defectives, however, there is the ever present possibility of 
dissipating the elements characteristic of the ideal family type, of ming- 
ling them with others not so productive or desirable. The tendency is 
by no means so strong as it is in out-breeding, but it is stronger than 
in inbreeding. It serves again to emphasize the fact that any system of 
breeding must be based upon matings of superior individuals. 

Out-breeding. — Out-breeding is merely a system of breeding best to 
best, at the same time avoiding relationship in the animals which are 
mated. While it may tend to avoid completely the disasters which often 
attended inbreeding, it is subject to all the defects of the old system of 
phenotypic selection. Chief among these is its tendency toward lack 
of uniformity in the herd. The harm which it does, however, depends 
largely upon the breed in which it is practised. Thus among Shorthorns 
the extraordinary multiplication of individuals of certain families leaves 
a wide field for the selection of superior individuals distantly related and 
of the same type, so that in this breed, a form of out-breeding which is 
really not out-breeding at all, but a very mild form of line-breeding, 
may be adopted without much danger. Out-breeding, however, is in a 
sense a harking-back to methods which have been discarded, and although 
the new breeder may do well to start his operations by avoiding too close 
affinities, he should steadily endeavor to master the problem of dealing 
with consanguineous matings sanely and effectively. 

Other Systems of Breeding. — Under the chapter on the utilization 
of hybrids in animal breeding, we have discussed at some length grading 
and cross-breeding. The former of these methods of breeding provides 
a simple and practical method for improving livestock on a large scale, 
and its practice is to be commended. Grading is not to be contrasted with 
any of the systems of breeding which have been described, but it may be 
compared on the one hand with pure breeding and on the other hand with 
aimless scrub breeding. In grading, any of the systems of breeding 



584 GENETICS IN RELATION TO AGRICULTURE 

which have been discussed above may be used, the only requirement is 
that the sire must always be pure-bred and of the same breed. By rigid 
selection of females which approach most nearly to the ideal type of the 
breed from which the sire is selected, grade herds after three or four gene- 
rations will approach verj^ nearly to the standard of excellence from an 
economic standpoint at least of pure-breds. 

Crossbreeding we have also described in a previous chapter. It is 
an economic procedure entirely, and is based on the uniting of favorable 
characteristics of two strains in the cross-bred animals. Along with 
crossing sometimes comes the increased vigor of hybrids, sometimes 
striking, other times only slight. Although greatly decried by breeders 
and advocates of pure-bred livestock, crossbreeding is sound in theory 
and productive of good results in practice. To reap its benefits, however, 
it must be followed systematically. The breeder must not be tempted 
to allow the excellence of cross-bred animals to overcome his better 
judgment to the extent of permitting their retention in the breeding herd. 
Increased vigor and size are not alone responsible for the adoption of cross- 
breeding by some livestock men, but the changing standards of market 
demands have sometimes favored types of livestock not represented 
in any existing breed. Two alternatives are then open to the breeder, 
to establish within existing breeds the type demanded or as it were to 
synthesize such a type by crossbreeding. The former method is pro- 
ductive of the most permanent good, but it is a slow and expensive pro- 
ject and one requiring the good judgment of an unusually critical breeder. 
It has its illustrations, however, in the establishment of the Cruick- 
shank family of Shorthorn cattle, the American type of Hereford cattle; 
and as an outgrowth of crossbreeding in the building up of the Corrie- 
dale sheep of New Zealand. Crossbreeding, however, often achieves 
the same result immediately with existing materials; and, the advantage 
of a particular cross having been established, it does not require as much 
skill in operation as the establishment of a pure breeding type. As 
agricultural science develops we may expect to see crossbreeding for 
specific purposes much more fully utilized than it is at the present time. 

Genotypic Selection. — -The method of genotypic selection is a method 
based on a knowledge of the genotypic constitution of the individuals 
used in mating. Although but little breeding can be ordered along this 
line on account of the dearth of knowledge of the actual factors which are 
concerned in particular character complexes, nevertheless to all practical 
purposes intelligent application of the methods of line breeding and in- 
breeding amounts to the same thing. Thus far our knowledge of factors 
is only extensive enough to apply this method of breeding to relativel}'- 
simple problems, such as that of producing polled breeds of cattle by the 



METHODS OF BREEDING 585 

use of polled mutants, mule-footed breeds of hogs, hornless sheep, or 
particular coat colors in horses, cattle, and swine. Nevertheless it is a 
very useful conception to add to the stock-breeder's fund of knowledge. 
The method of breeding for increased fecundity in poultry devised by 
Pearl is the best existing illustration of the employment of genotypic 
selection in attacking a problem of economic importance. We have 
pointed out how Pearl on the basis of investigations of winter egg pro- 
duction in fowls established the fact that two dominant factors for high 
winter egg production existed. One of these factors, L, determines the 
production of pullets which lay somewhat less than thirty eggs during the 
winter period; the other factor, M, which is sex-linked, adds to this so 
that birds possessing both these factors lay over thirty eggs during the 
winter cycle. The breeder's problem, therefore, starting with a mixed 
flock, is to isolate and breed from individuals of the genetic constitutions 
(WM){WM)LL for males and {WM)ZLL for females, to the end that the 
flock will consist entirely of individuals of these genotypes. So valuable 
are the specific directions which Pearl has given that they are printed in 
full below. 

1. Selection of all breeding birds first on the basis of constitutional vigor and 
vitality making the judgment of this so far objective as possible. In particular the 
scales should be called on to furnish evidence, (a) There ought to exist, for all 
standard breeds of fowls, normal growth curves, from which could be read off the stan- 
dard weight which should be attained by a sound, vigorous bird, not specially fed 
for fattening, at each particular age from hatching to the adult condition. These 
curves we shall sometime have, (b) Let all deaths in shell, and chick mortality, be 
charged against the dam, and only those females used as breeders a second time which 
show a high record of performance in respect to the vitality of their chicks, whether 
in egg or out of it. This constitutes one of the most valuable measures of constitu- 
tional vigor and vitality which we have. If for no other reason than to measure their 
breeding performance, a portion of the females each j^ear should be pullets. In this 
way one can in time build up an elite stock with reference to hatching quality of eggs 
and viability of chicks, (c) Let no bird be used as a breeder which is known ever to 
have been ill, to however shght a degree. In order to know something about this, 
why not put an extra leg-band on every bird, chick, or adult, when it shows the first 
sign of indisposition? This then becomes a permanent brand, which marks this 
individual as one v.'hk-h failed, to a greater or less degree, to stand up under its environ- 
mental measures of constitutional vigor, (d) Many of the bodily stigmata by which 
the poultrjauan, during the last few years, has been taught to recognize constitutional 
vigor, or its absence, have, in my experience, little if any real significance. Longevity 
is a real and valuable objective test of vigor and vitality, but it is of only limited 
practical usefulness, because of the increasing difficulty with advancing age of breeding 
successfully on any large scale from old birds of the American and other heavy 
types. 

2. The use as breeders of such jemales only as have shown themselves by trap- 
nest records to be high producers, since it is only from such females that there can be 
any hope of getting males capable of transmittmg high-laying quaUties. 



586 GENETICS IN RELATION TO AGRICULTURE 

3. The use as breeders of such males only as are known to be the sons of high- 
producing dams, since only from such males can we expect to get high-producing 
daughters. 

4. The use of a pedigree system, whereby it will be possible at least to tell what 
individual male bird was the sire of any particular female. This amounts, in ordinary 
parlance, to a pen pedigree system. Such a system is not difficult to operate. In- 
deed, many poultrymen, especially fanciers, now make use of pen pedigree records. 
It can be operated by the use of a toe-punch. All the chickens hatched from a par- 
ticular pen may be given a distinctive mark by punching the web between the toes in 
a definite way. If one desires to use a more complete individual pedigree system, he 
will find the system described in Bulletin 159 of the Maine Agricultural Experiment 
Station a very simple and efficient one. It has been in use at this Station for 7 
years, with entire satisfaction, on the score of both accuracy and simplicity. 

5. The making at first of as many different matings as possible. This means the 
use of as many different male birds as possible, which will further imply small matings 
with only comparatively few females to a single male. 

6. Continued, though not too narrow inbreeding (or line-breeding) of those lines 
in which the trapnest records show a preponderant number of daughters to be high 
producers. One should not discard all but the single best line, but should keep a 
half dozen at least of the lines which throw the highest proportions of high layers, 
breeding each line within itself. 

In the above set of directions two things will challenge the student's 
interest most, namely the emphasis which is laid upon constitutional 
vigor and vitality in the selection of breeding birds, and the fact that a 
system of line-breeding or inbreeding is used in order to increase 
fecundity. 

The relation of the above directions to the genotypic behavior are 
not difficult to point out. Of females there are two different types 
{WM)ZLL and {WM)ZLl which are high producers; the remainder are 
either mediocre or low producers. It is assumed that by trap-nest 
records, it has been possible to segregate out a certain number of such 
high-producing hens from a mixed flock of low, high, and medium pro- 
ducers. When these are mated to males from the same lot, a variety of 
results will be produced according to the genetic constitution of the males. 
In Table LXXIX are collected the results which follow when females of 
the two high-producing genotypes are mated with the nine possible kinds 
of males. Now if the numbers of females of genotypes (WM)ZLL and 
(WM)ZLl in each pen are approximately equal — in practice those of geno- 
type {WM)ZLl would probably be in excess — then it will be practically 
impossible to distinguish matings of types (1) to (3) and possibly (4) and 
(5) unless the number of daughters tested from each pen be relatively 
large. In this connection we recall the fact, as a further difficulty, that 
modifiability in egg production is relatively very great. If now an 
equal number of pens from matings (1) to (5) should happen to have 



METHODS OF BREEDING 



587 



TO 




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588 



GENETICS IN RELATION TO AGRICULTURE 



been chosen, then we will obtain the following distribution of males 
with respect to their genotypes and relative numerical frequencies: 

(1) {WM){WM)LL 27 

(2) {WM){WM)Ll 30 

(3) {WM){WM)ll 7 

(4) {WM){Wm)LL 9 

(5) {WM){Wm)Ll 6 

(6) {WM){Wm)ll 1 

Total 80 

In the next generation, therefore, the probabilities are strongly in 
favor of the selection of males of types (1) and (2) and the trap-nest 
records should insure the selection of hens which are mostly of the two 
genotypes given in the table. At any rate matings will be restricted to 
types (1) to (6), the inferior types (7) to (9) are excluded. Accordingly 



9D39 {62) X d'Dbi 



9 E232 {69) X &555 - 








9 GIO {100) 




9F255 {48) X c^564 < 


14 {99) 

88 {23) 

254 {16) 

9G12 a^)- 

19 {70) 
39 {100) 




303 {64) X &663 < 


53 {44) 

81 {34) 

85 {73) 

192 {57) 


237 {65) X &554 < 


136 {48) 


213 {29) 


Mean, 62 




9G18 {61) 
27 {83) 
46 {116) 




347 {69) X <^562 < 


196 {56) 
211 {36) 
248 {67) 

9 Gil {47) 
134 {111) 




363 {74) X 6^567 


165 {35) 
198 {39) 




Mean, 61 


506 {19) 

Mean, 57.74 



Fig. 226. — Pedigree of line D5DZ9, characterized by high winter egg production. 
Bold faced figures are band numbers of females, italics of males. Italics in parentheses 
give the winter egg records. (After Pearl.) 



METHODS OF BREEDING 589 

this second generation should show a very marked improvement in egg 
production, if breeding be carried on within the Une. 

9F308 {78) c? 554-0 



9 E248 (47) X ^553 
{Mutant?) 



354 {55) X &566 < 



9 G30 {46) 
62 {68) 
117 Ue) 
197 {66) 
428 (45) 
495 {1) 
Mean of high line, 
62.8. 



166 i49)Xd'D31 



141 {51)Xd'D31 



9 G204 (41) 
254 (/5) 

/ 9 G229 {28) 
\ 458 {11) 

Mean of DSl's 
daughters, 23.75. 



9D168 {33)X&D61 ■ 



172 {50) X& 57 8 -Cross 
Mean of muta?tt 
{high) line, 66.5 



9 E231 {25) X 6^552 - 9 F233 {32) X cf 573-0 

( 9 G221 {16) 
419 {9) X (^551 - 9 F165 (7) X d^669 430 (;^) 

I 477 {1) 

209 (55) X c?555- Mean 0/ viain 

{low) line, 9.67. 
/ 9F250 {20) 



313 (^e) X cf 554 



363 {11)X&550 



\ 174 (£i) 

- 9 F249 (SO) 
Mean of main 
{loio) line, 22.0. 



15 {18) 
163 (i5) 
200 {12) 
141 (0) 
116 {28) 
151 (H) 
24 (^S) 
Mean of main 
{low) line, 17.6. 

Fig. 227. — Pedigree of line D61D168, characterized by low winter egg production. 
Conventions are the same as in Fig. 226. The progeny of the mutant (?) high producer 
£^248 is included in this pedigree. {After Pearl.) 

This method of increasing egg production is not entirely thecyetical, 
but it has actually worked after 9 years of patient selection of high 
trap-nest performers failed to show any improvement. The two pedi- 



590 GENETICS IN RELATION TO AGRICULTURE 

grees given herewith show the type of results which Pearl has secured 
by an appHcation of this method of breeding. In Fig. 226 is given the 
pedigree of line Z)5Z)39, a high-producing Hne. It will be observed that 
a mean winter egg production of about sixty eggs is maintained through- 
out four generations. In every case, of course, males were selected for 
mating of a genotype corresponding to the females. The last generation 
in this pedigree shows how large a range of phenotypic fluctuation may be 
expected in breeding operations with fowls. 

For contrast with the above hne we may present line D61D168, a low- 
producing line. The pedigree is given in full in Fig. 227. Here the 
mean winter egg production of the main low line is about one-third that 
of hne Z>5Z)39. This hne is interesting on account of the appearance of 
a high-producing individual, E2A.S, which is probably a product of 
Mendehan segregation, but may possibly be a mutant. Not only was 
this individual herself a high producer, but she transmitted her producing 
abilities to her daughters, so that in the high line of this race, only one 
individual (t495, possibly pathological, failed to exhibit a considerably 
higher winter egg production than the highest individual in the low por- 
tion of the line. These two pedigrees illustrate clearly what a systematic 
plan of breeding may accomplish after mere phenotypic selection has 
failed completely. 

We should not fail to point out as a factor to be considered in in- 
terpreting Pearl's directions for breeding poultry for high fecundity, that 
neither Pearl nor any other scientist claims that the two factors, L and M, 
are the only ones concerned in breeding for fecundity. In any mixed 
flock of birds there must be a number of other factors, which although they 
may not have as marked an effect as the two primary factors, nevertheless 
will appreciably affect winter egg production. For this reason the 
breeder is admonished to use moderate inbreeding or line-breeding, 
because by too close inbreeding he may inadvertently breed his flock 
to a homozygous condition for unfavorable modifiers. Line-breeding 
is necessary, because by this method a like genotypic constitution is 
assured. Further, too hasty rejection of lines which do not measure 
up to standard may result inadvertently in the discarding of some line 
which had greater potentialities than those at first most productive, 
consequently the warning not to reject all but the best line. Moreover, 
we suspect that by isolating different high lines and then crossing them 
and applying the above procedure to the hybrid progeny still better 
strains might result. The directions which Pearl has given for poultry 
breeding may be applied with proper modifications to other hvestock. 
They should be carefully studied by every breeder, with the distinct 
proviso that no rule of thumb, however excellent, can supply the ability 
for intelhgent practical application, an indispensable feature of successful 
breeding operations. 



CHAPTER XXXVIII 
METHODS OF CONDUCTING BREEDING INVESTIGATIONS 

Any livestock breeder who wishes to carry on breeding operations in 
an intclhgent fashion, particularly if on a large scale, will find it necessary 
to adopt some definite system of keeping records. Whatever system is 
adopted it should fulfil at least three requirements: it should be simple; 
it should be concise, that is it should confine itself to the essential features 
of the breeding operations; and it should be adapted to the particular 
conditions of the individual livestock breeder. The last desideratum 
makes it impossible to outhne here any specific plan for keeping written 
records, consequently certain features of this problem will be discussed 
so that some definite conception may be gained of the matters with which 
records should deal. 

Judging the Individual. — Of first importance in breeding operations 
is some method of determining individual worth. In certain cases, as 
for example, in beef cattle, this depends largely upon visible character- 
istics, and the breeder has only to build up in his mind by constant 
association with his Hvestock an ideal to which he desires to direct 
improvement in his herd. Whenever he can introduce objective tests, 
the breeder gains by doing so. The practical breeder has often felt the 
need of such objective standards of judgment, and from time to time he 
has attempted to introduce them. Sometimes such tests are very easy 
to apply, as for instance the speed test in race horse breeding. Some- 
times, however, they are more difficult of utilization, as for example, 
individual butter-fat production in dairy cattle or individual egg pro- 
duction in poultry. Nevertheless even such records may be obtained 
economically if everything be planned so as to expedite the work con- 
nected with them. Methods of keeping dairy records have been devised 
which enable the dairymen to obtain and record accurately the daily 
production of his cows by spending about 2 minutes per day per cow 
in doing it. In Fig. 228 is shown the equipment for carrying out such 
work and a convenient mode of arranging it. It will be noted that all 
the necessary equipment is at the hands of the operator, so that no time 
whatever is lost in obtaining and recording the data. For testing 
butter fat, composite samples are used and the actual test is often made 
by some central creamery or appointed milk tester, rather than by the 
dairyman himself, although the latter method is perfectly feasible. 

591 



592 



GENETICS IN RELATION TO AGRICULTURE 




Fig. 228. — Equipment necessary for obtaining individual records of production of dairy- 
cows. (After Lane.) 




Fig. 229. — Type of trap-nest used in Maine station poultry breeding operations, shown 
set and sprung. One side removed to show interior. (After Pearl and Surface.) 



CONDUCTING BREEDING INVESTIGATIONS 593 

Since so much has been said about the Maine Station investigations 
of fecundity in fowls, perhaps it would be of some interest to know how 
records are obtained there. The type of trap-nest in use is shown in 
Fig. 229. Details of construction need not be taken up here, except to 
remark that durability of materials is a prime requisite for continuous 
service. The absence of any springs or other involved contrivances has 
made it possible to use this type of trap-nest in extensive breeding in- 
vestigations involving a flock of about 2000 hens. Ten such nests are 
used in a pen of fifty birds, and an attendant visits the pens at intervals 
of one hour or more, depending upon the rate of egg laying. Obviously 
a method such as this is expensive even when reduced to the simplest 
terms, and it is, therefore, applicable only to the selection and production 
of breeding stock. It is difficult, however, to conceive of any other 
accurate criterion which might be adopted. 

It should be noted that statistical requirements do not demand that 
complete records be obtained, for the existence of modifiability and other 
kinds of individual variability make it impossible in any event to get 
anything but an approximate record. Accordingly in recording the data 
of production of dairy cows, for example, it is not necessary to weigh and 
test the milk every day for the whole period of lactation, but two or 
three 7-day periods at stated times with respect to the beginning 
of lactation will give a sufficiently accurate estimate for all practical 
purposes. Similarly in poultry breeding. Pearl has found that produc- 
tion during the winter period is a sufficiently accurate and distinctive 
index of the egg-laying capacity of a hen. 

Further the danger from unjust comparisons should always be empha- 
sized. A comparison between egg production of hens in the second 
laying season and pullets would favor the pullets, for pullets ordinarily 
lay more eggs during the first season than they do as hens in the second 
season. Moreover different parts of a given season are not equivalent. 
A pullet lays more eggs in a given length of time during the spring cycle 
beginning about March 1, than she does during the winter cycle. A cow, 
likewise, produces more milk during the early part of her lactation period 
than she does later on, and she reaches her maximum capacity at 5 
or 6 years of age. With respect to these points we have reproduced 
in Table LXVI, the comparative indices which Pearl has calculated and 
which provide a method of comparing the productions of cows of different 
ages at different stages in the lactation period. As an additional variable 
in this case we should include the time at which a cow freshens, whether 
in spring, summer, or fall, as having a definite influence on herd produc- 
tion of milk and butter fat. We could go on recoimting without end 
such factors which must be considered in making accurate comparisons. 
The point, however, is sufficiently obvious, namely, that even objective 

38 



594 



GENETICS IN RELATION TO AGRICULTURE 



Tilly Alcartra 123459 
Calved October 2, 1908 

Butter 7 days (6 years) 30 . 20 

Milk 632.30 

Butter 7 days (5 years) 29 . 27 

Milk 715.40 

Butter 30 days 122.71 

Milk 3,066.80 

Butter 90 days 360 . 07 

Milk . 8,793.90 

Butter 100 days 396 . 83 

Milk 9,702.80 

Butter 7 days (8 months after 

calving) 19.23 

Milk 473 . 10 

Butter 1 year 1,189.04 

Milk 30,451.40 

(World's yearly milk record) 

Butter 7 days (3 years) 23 . 15 

Milk 613.00 

Butter 7 days (8 months after 

calving) 17 . 06 

Milk 420 . 00 

Butter 1 year 841 . 23 

Milk 21,421.30 

Butter 7 days (30 months) 17 . 39 

Milk 490.40 

Butter 7 days (8 months after 

calving) 14 . 35 

Milk 362.10 

Butter 285 days 556.20 

Milk 14,837.20 



I 

Alcartra Polkadot Corrector 30624 . fl 

22 A. R. O. daughters: ■ 

Geneseo Belle Polkadot 34 . 39 

Milk 733.60 

Butter 1 year 916.17 

Milk 20,816.20 

Alcartra Abbekirk 27.87 

13 others from 20 to 27 . 1 pounds 
10 A. R. sons 

He has 873^^ per cent, of the same 
blood as OUie Watson Prima 
Donna, 31.10. 
Brother to the dams of: 

Hilldale Segis 33.17 

K. P. Alcartra {S}^4 years) 30.87 4 

Butter 30 days 121 .29 f 

(World's 3-year-old records) 
By a brother to the sires of: 

Sadie Vale Con. 4th 41 .06 

Four others from 30 to 31.8 
pounds. 



^ 



Tilly Lou 2d 82057 

By a brother to the sire of six 30- 
pound cows. 

Her sire is by a son of De Kol 
Burke, whose 73 A. R. O. daugh- 
ters include: 

Lint Burke (4 years) 32 . 7fi .i 

River Sadie D. K. Burke 32.29 ' 

Milk 7 days 920.80" 

Milk 30 days 3,725.60 

Milk 2 years 54,805.20 . 

(World's milk records) 
. Five others from 30 to 31.7 pounds, 
28 others from 20 to 27.2 pounds; 
and who is grandsire of Sp. 
Brook Bess Burke, 34.51 pounds. ** 

Five others from 30 to 33.5 ( 

pounds. 



FiQ. 230. — Pedigree of Tilly Alcartra, world's record milk producer, showing produf- 



CONDUCTING BREEDING INVESTIGATIONS 



595 



Chief Phiebc Oak Duchess 28176 
21 A. R. O. daughters: 

OlHe Wat. Prima Donna 31 . 10 

Lilhe Geurima 2d 27.46 

Wisconsin Bride Phiebc 27 . 42 

Alhe Nig. 2d 25.98 

Pet Douglas 2d 23.02 

Seven others from 20 to 22.9 
pounds, 5 A. R. sons. 
From a sister to the dams of: 

Grace Fayne 2d's Hom 35 . 55 

(World's record) 

Jessie Fo. 2d's Maud Hom 31 . 17 

Three others from 30 to 31 
pounds. 
Alcartra Polkadot 50798 

Butter 7 days 29.09 

Milk 597.10 

Butter 30 days 120. 16 

Milk 2,605.00 

Five A. R. O. daughters: 

Lyndon Al. Polkadot 32 . 54 

Al. Polkadot Ormsby 31 .25 

Al. Polkadot 2d (3>^ years) 22.97 

Two A. R. sons. 

Sister to the sire or dams of: 

Sp. Brook Bess Burke 34.81 

Snowball Pink 3d 31 .69 

Ollie Watson Prima Donna 31.10 

Heilo Butter Boy Burke 29327 
12 A. R. O. daughters: 

Heilo Oak Burke 23.40 

Starlight Burke 20.43 

His sire is by a brother to the sires 
of: 

Urma Burke 35.21 

Prin. Hengerveld D. K 33.62 

Blanche Lyons De Kol 33.31 

Blanche D. K. Hengerveld 33 . 20 

Bloom. Heng. Edith 32.45 

Crown Pontiac Josey 32 . 34 

Frenesta Heng. D. K 32.20 

11 others from 30 to 32 pounds. 

Tilly Lou 62052 

Her sire is by a brother of the sire of: 

Pauline Alexis (10 years) 32 . 40 

Milk 645.20 

• Butter 30 days 128.35 

Milk 2,629.50 

Butter 60 days 238 . 12 

Milk 5,225.10 

By a brother to a grandsire of: 

Maplecrest Pon. Girl (4 j^ears) 35. 15 

Maplecrest Pon. Highlawn (4 years) 30 . 72 

Hattie D. K. Colantha 30 . 54 

Burton High. 2d's Har 30 . 14 

and to the grandam of: 

El. Barn. Mech. D. K 30 . 49 



f Phiebe De Kol Burke 25368 
28 A. R. O. daughters: 

Maude Burke 32.03 

W. R. Jones 2d'3 Phiebc 30. 18 

She will do Unecda 26.36 

Five others from 20 to 25.6 pounds. 
17 A. R. sons. 
Lady Oak 2d 39947 

Butter 7 days 21 . 49 

Milk 492.90 

Five A. R. O. daughters: 

Oak De Kol (10 years) 31 . 54 

Lady Oak 2d's Hom. D. K 30 . 17 

Two others with 21 and 27.4 pounds. 
Two A. R. sons. 
Pearl of the Dairy's Joe De Kol 23450 
75 A. R. O. daughters: 

Pearl Ormsbv Burke 30.56 

Pearl Neth. Vcrgeus 28.00 

32 others from 20 to 26.9 pounds. 
9 A. R. sons. 
Alcartrii's 2d's Rose 44430 

Butter 7 days 17.80 

. Milk 388.90 

Three A. R. O. daughters: 

Alcartra Polkadot 29.09 

Butter 30 days 120. 16 

Alcartra Peach 20.42 

Two A. R. Sons. 
Phiebc De Kol Burke 25368 

His sire is by a brother to the sire of 

Aaggie Cornu. 
Pauline (4^ years), 34.32 pounds; 

the first 34-pound cow. 
Four others from 30 to 33.2 pounds. 
Heilo 4th's Pet 43611 

Butter 7 days 24.92 

Milk 632.90 

Two A. R. O. daughters, including 
Heilo Queen De Kol (33 months) 
15.60 pounds butter fat, 309.70 
pounds milk. 
One A. R. son. 
lolena Fairmount's Statesman 

13 A. R. O. daughters, including 
lolena Margaret, 23.54 pounds 
butter fat, 472.80 pounds milk; 
Wait-A-Bit, 19.88 pounds butter 
fat, 461.10 pounds mQk; Angos- 
tura, 19.02 pounds • butter fat, 
446.50 pounds milk. 

Ophelia Diana 46996 22.80 

One A. R. O. daughter: 
A daughter of Eunice Clay's Sir 
Henry and Ophelia Rose 2d. 



a and breeding performances of her ancestors. {From Kimball's Dairy Farmer.) 



596 GENETICS IN RELATION TO AGRICULTURE 

data must be handled intelligently. In this connection the need of 
additional comparative tables like Table LXVII for other characters and 
other classes of Hvestock should be mentioned. They are not difficult 
to obtain and undoubtedly they will be available some day 

Pedigrees.^ — The pedigree of an animal is simply a record of its 
ancestry, and accordingly the ideal system of recording pedigrees is 
that system which gives proper emphasis to each animal in the pedigree. 
The one-time fashionable practice of tracing pedigrees back through 
five or six or even more generations to some illustrious sire or dam 
cannot, therefore, be too strongly condemned, for it over emphasizes 
remote ancestors in certain lines and tends to underrate the importance 
of a possibility of inferiority in nearer ancestors. As a test of purity 
of blood, the Arabians require that their horses trace through long lines 
of descent to the five mares of Al Khamseh; there is, however, no justi- 
fication for this practice in modern breeding operations. 

The pedigrees of pure-bred breeding stock are recorded in herd 
books. For such animals it is only necessary to consult the herd books 
in order to trace out their ancestry. However, it is usually more con- 
venient, since the pedigree must be traced through several volumes of 
the herd-book, to record it in extended form in the herd record. This 
is not a difficult task; it need be done only once for every animal, and 
the task is still further lightened by the fact that the individuals of any 
established herd will have so many common ancestors that they will 
duplicate one another's pedigrees to a great extent. It is, however, 
necessary to say a word regarding the method of recording such pedigrees. 
The following pedigree of Roan Gauntlet, a famous old Cruickshank 
Shorthorn sire, taken directly from Volume XXII. of the "American 
Shorthorn Herd-book," illustrates a method of recording pedigrees which 
should not be followed by breeders: 

Roan Gauntlet 45,276 (35,284).— Roan, calved May 19, 1873, bred by A. Cruick- 
shank, owned by Mr. Rennie, got by Royal Duke of Gloster (29,864), out of Princess 
Royal by Champion of England (17,526) — Carmine by The Czar (20,947) — Cressida 
by John Bull (11,618)— Clipper by Billy (3151)— by Dandy (6918)— by Tiptop 
(7633)— bred by Mr. Mason. 

The reason why this method should not be followed may be seen 
very easily in Fig. 232, which illustrates a proper way of recording a 
pedigree. Here the bold-faced type indicates those animals which were 
included in the pedigree as given in the herd-book. Of the sixty-two 
ancestors of Roan Gauntlet in five generations only nine are included in 
the herd book record. Further the record is defective in that it fails to 
give any evidence of the type of breeding which was employed in pro- 
ducing Roan Gauntlet. The way this bull traces back to the great 



CONDUCTING BREEDING INVESTIGATIONS 



597 



Cruickshank bull Champion of England is the striking feature of his 
pedigree. 

The criticism of the above pedigree is not, it should be clearly under- 
stood, directed at the method of recording pedigrees in the American 
Shorthorn Herd-book, although it is a fair statement to make that the 
method that has since been employed of recording simply the name of 
sire and dam is more economical and just as satisfactory. Even by the 
old method, however, the pedigrees are so recorded that the entire set 
of ancestors may be determined. The point, however, is simply this, 
that such pedigrees should not be used as standards of judgment of 
ancestry, but rather those of the type shown in Fig. 230. 




Fig. 231. — Tilly Alcartra. No. 123459, Holstein. Production for one year, 30451.4 lb. 
milk containing 951.2 lb. butter fat (average test 3.12 per cent.). 

The addition of other data to the pedigree indicative of the value 
from a breeding or productive standpoint of the animals therein listed 
adds greatly to its value, particularly to the new breeder who is not yet 
fully famihar with the great names of breed history. The pedigree of 
Tilly Alcartra 123,459, the record-breaking Holstein-Friesian cow por- 
trayed in Fig. 231, is given in Fig. 230 along with data relative to the 
performance and breeding value of the animals whose names appear 
in the pedigree. A pedigree worked out like this one is a much safer 
guide in judging merit than one which gives data proving that the animal 
in question traced in the fourteenth generation three times to some 
famous sire of ancient history. Performance should be insisted upon all 
along the hne, and when three or four generations of some subdivision 
in a notable hne fail to bring forth performing individuals, it is high time 



598 



GENETICS IN RELATION TO AGRICULTURE 



for the breeder to suspect that something has been lost in that Hne of 
descent, something that a pedigree, however excellent in remote ancestors, 
cannot supply. 

The Coefficient of Inbreeding. — We would call attention to Pearl's 
coefficient of inbreeding as an instance of another refinement which has 
been advocated for use in practical breeding operations. Pearl proposes 



Roan 
Gauntlet 
45276 

(35284) 



Royal Duke 
of Gloster 

20901 

(29864) 



Al 



Princess 
Royal 



Grand Duke 
of Gloster 
19900 (26288) 



Mimulus 



A2 



Champion of 
England 

(17526) 



[ Carmine 



©Champion 
of England 
(17526) 



Duchess of 
Gloster 9th 



£ Champion 
of England 
(17526) 



Mistletoe 



f Lancaster 
Comet 
(11663) 



[ Virtue 



' The Czar 

(20947) 



[ Cressida 



X Lancaster 

Comet 

(11663) 
X Virtue 

Lord Raglan 
(13244) 

Duchess of 
Gloster 6th 



X Lancaster 

Comet 

(11663) 
X Virtue 

®Lord Raglan 
(13244) 

Maidstone 

A4 

The Queen's 
Roan (7389) 



X Queen's Roan (7389) 
X 



/ X Plantagenet 
I Verdant (11906) 
/ Crusade 7938 
\ Brenda 

Lord Garlies 20236 

(14819) 

Duchess of Gloster 

X Queen's Roan (7389) 



[ Plantagenet 
(11906) 



/ X Plantagenet (11906) 
1 X Verdant 
/ X Crusade (7938) 
1 X Brenda 

r Matadore (11800) 
\ Phantassie 
A5 
Will Honeycomb (5660) 



©Will Honeycomb (5660) 
Lupin 

The Duke of Lancaster, 
10929 
Madaline 



1, Verdant f The Exchequer (9721) 

I Prigg 

r ©Lord Raglan / X Crusade (7938) 



I (13244) 
1 Corianda 



John Bull 

(11618) 

Clipper 



\ X Brenda 

The Baron (13833) 
Czarina 

Rumous (7456) 
Mayoress 



Billy (3151) 



Fig. 232. — Pedigree of Roan Gauntlet, a famous Cruickshank Shorthorn bull. 



that inbreeding be used in a generic sense to include all cases in which 
some of the matings in the pedigree were of related individuals. In 
order to indicate degree of inbreeding he suggests the use of a coefficient 
of inbreeding of the form 

100 (P„+i- <?„ + i) 



Zn — 



Pu 



+ 1 



which is essentially a mathematical expression of the relation in per 
cent, between the maximum number of different ancestors which an indi- 



CONDUCTING BREEDING INVESTIGATIONS 599 

vidual might have in a given generation, and the amount of repetition 
which has occurred in its ancestry. 

In the above formula for Z, the coefficient of inbreeding, P„+i 
denotes the maximum possible number of different ancestors which an 
individual might have in matings of the (w + l)th generation and 
Qn+i, the actual number of different ancestors which he has. For an 
application of this coefficient we take the pedigree of Roan Gauntlet as 
given in Fig. 232. It gives the following series of values for Z : 

100 (2 - 2) 
Zo = n = per cent. 

100J4-4) 
Zi = 2. ~ per cent. 

100(8-6) 
Zo = o = 25 per cent. 

100 (16 - 10) „_ ^ 

Z3= 1-^ = 37.. 5 per cent. 

„ 100 (32 - 19) ^_ ^,^^ 

Z4 = wn = 40.62.') i)er cent. 

To determine these values we have started with the A 1 generation in 
which pi = 2 and of necessity qi = 2 also. The value of Zo, therefore, 
must be in all cases. In the A2 generation p2 = 4 and qo = 4, also, 
because all these four animals are different and have not previously 
appeared in the pedigree. The value for Zi, therefore, is 0. In genera- 
tion A3, pz = 8. Champion of England appears twice in this genera- 
tion, and since he has appeared already in A 2, the two reappearances 
in A 3 are crossed out. Counting the remaining individuals in this genera- 
tion, we find qs = 6, and consequently Z2 = 25 per cent. Now this Z2 = 
25 signifies not only that Roan Gauntlet in the third ancestral generation 
has 25 per cent, less than the maximum possible number of different 
ancestors, but also that in any generation further removed he must of 
necessity have at least 25 per cent, less than the maxinuim possible 
number of ancestors. In the next following generation, Ai, pi = 16. 
In determining g„+i we strike out Lancaster Comet and Virtue, sire 
and dam respectively, of Champion of England. It is worth while noting 
here that these two animals are automatically eliminated in this genera- 
tion because of the reappearance of an animal in a lower generation in this 
same line of descent. Reappearances at the apex of a line of descent are 
called primary reappearances and are marked ® in this pedigree, whereas 
reappearances which are determined by the primary reappearance of 
an individual in a lower generation are called secondary reappearances 
and they are marked with the sign X . It is only necessary to determine 
primary reappearances in calculating the coefficient of in])reeding, for 
secondary reappearances may be accounted for by simply doubling the 
total number of reappearances in the next lower generation. Continuing 



600 



GENETICS IN RELATION TO AGRICULTURE 



down the A 4 generation we meet Avith Lord Raglan as a primary reappear- 
ance as the sire of Mistletoe and further down as the sire of The Czar, also. 
The total number of primary and secondary reappearances in A 4 is, 
therefore, 6; and since the expression 2?„+i — gn+i is merely a measure 
of the total number of reappearances, the value of Z3 = ^i6= 37.5 
per cent. In ils we know the total number of secondary reappearances 
will be 6 X 2 = 12. There is one primary reappearance, that of Will 
Honeycomb, which must be added to this value, making the total number 
of reappearances in this generation thirteen. This gives the value 
Z4 = 40.625 per cent. If we have, therefore, at hand an extended 
pedigree of an animal it is a simple matter by this method to determine 
its coefficient of inbreeding for any number of generations. 



6 40 



20 





^ 






'^^-'^'' 










fs/A 


v/ 1/ 
CocV 








■ 








/ 
/ 
/ 












h 
/ 


'// 














11 















6 8 10 

Generations 



12 14 



Fig. 233. — Curves of inbreeding: B X S, continued brother X sister matings; P X O, 
continued parent X offspring matings; C X C^, continued double-cousin matings; C^ X C^, 
continued single-cousin matings. Continued matings of uncle X niece give a curve iden- 
tical with Ci X C'l. {After Pearl.) 

In Fig. 233 are shown a number of curves of inbreeding which show 
graphically the rate of concentration of blood hues with different types 
of matings. Continued brother X sister matings give the maximum 
values for the coefficient of inbreeding. In this connection Pearl calls 
attention to the similarity of form of the brother-sister and double cousin 
curves and of the parent-offspring and single cousin curves. 

The Coefficient of Relationship. — Obviously it is necessary for 
determining the significance of the coefficient of inbreeding to know how 
the reappearances occur in the pedigree. Thus if animals appear on 
both sire's and dam's line of descent sire and dam are related in some degree. 
But it is possible as Pearl points out to have a high coefficient of inbreed- 
ing without any relationship whatever between sire and dam. In fact, 
specifically the limiting value of the coefficient of inbreeding where sire 



CONDUCTING BREEDINC INVESTIGATIONS GOl 

and dam are totally unrelated lags only one generation behind the value 
for continued brother-sister matings. Pearl, therefore, proposes to de- 
termine not only the coefficient of inbreeding, but also a coefficient of 
relationship which shall express mathematically the degree of kinship 
existing between an individual's parents. We again take the pedigree of 
Roan Gauntlet as an illustration of the method of calculation employed. 
We obtain the following series of values: 



Zo = 


iv'i = 


Zi = 


K^ =0 


Z. = 25 


/V3 = 50 


Zs = 37.5 


Ki = 75 


Zi = 40.6 


K, = 75 



The values for K, the coefficient of relationship, were determined in the 
following fashion. In ^3 on the sire's side. Champion of England which 
has already appeared on the dam's side reappears twice. The maximum 
possible number of animals different from those on the dam's side in this 
generation is four. Since two of these are identical with an individual 
which has already appeared on the dam's side, Kz = "^"i = 50 per cent. 
In Ai the double primary reappearance of Champion of England in Az 
automatically determines a total of four secondary reappearances, 
and to these are added two primary reappearances of Lord Raglan. 
In Ai, therefore, K = ^g = 75 per cent. In A 5 there are no additional 
primary reappearances involving both sides of the pedigree, consequently'' 
the value of K remains at 75 per cent. It seems wise for breeders to use 
these coefficients in order to gain precision in the use of terms, if for no 
other purpose. 

Of course the use of inbreeding coefficients does not alter the prob- 
lem of inbreeding from a biological standpoint. That problem is 
concerned with the effect of mating closely related animals. It has 
already been pointed out that the coefficient of inbreeding may be high 
when there is no relationship between sire and dam as, for example, when 
a closely inbred Jersey cow is bred to a closely inbred Holstein-Friesian 
bull. Such matings are of course not a part of the problem of inbreeding 
as it is understood in practice. For a precise expression of this problem 
we must look to the coefficient of relationship. A coefficient of relation- 
ship of 50 per cent, for Az would probably be a fair mathematical require- 
ment for inbreeding as conceived in practice. A coefficient of relation- 
ship of this magnitude includes double cousin matings as well as those 
of brothers with sisters and parents with offspring, but this appears to be 
a fair inclusion, if reference be made to the curves of inbreeding given 
in Fig. 233. For further details of the applications of these coefficients 
reference must be made directly to Pearl's work. 



602 



GENETICS IN RELATION TO AGRICULTURE 



Marking Individuals. — The problem of marking individuals often is 
difficult where large numbers of individuals are involved. When very 
small herds are kept in which the animals may be known individually 
this matter is not very important, because the animals may simply be 
given a distinctive name, and any notes which it may be necessary to 
make may be recorded under that name. But when individuals become 
more numerous, it is usually necessary to have some safe and effective 
way of distinguishing them. For cattle aluminum ear tags of various 
kinds are often used, and these may be obtained stamped with any numbers 
which are desired. These may be used for smaller animals, also, or the 
ears may be punched in various fashions. A method used by Dr. 

Fig. 234. — Method of identifying sheep by holes punched in the ears. (From the Journal 

of Heredity.) 

Bell in sheep breeding experiments is illustrated in Fig. 234. By the use 
of eight holes with place values such as are indicated in the diagram, it 
is possible to identify 256 sheep according to the following combinations : 

Total sheep identified by hole 1 

Total sheep identified by 1 hole 8 

Total sheep identified by 2 holes 28 

Total sheep identified by 3 holes 56 

Total sheep identified by 4 holes 70 

Total sheep identified by 5 holes 56 

Total sheep identified by 6 holes 28 

Total sheep identified by 7 holes 8 

Total sheep identified by 8 holes 1 

This is a very simple mode of identification, and by means of a rubber 
stamp with a sheep's head outline or description sheets having such 
a head as that shown in Fig. 234 printed upon them, it is very easy to 
record accurately the designation which has been given to any particular 
sheep. The method can of course be used with other animals, and it 
avoids the difficulty of loss which sometimes is met with in using ear tags. 



CONDUCTING BREEDING INVESTIGATIONS 



603 



Poultry may likewise be marked in two different ways either with web 
punches or with aluminum bands which fit around the shanks. For large 
numbers the latter method is preferable. 

Recording Data. — The keynote of any system of recording data 
should be simplicity. This requirement must be met in scientific work; 
it is, however, particularly important in practical breeding for herdsmen 
have but Hmited time at their command for keeping records. 

The time necessary for recording data may often be very much cur- 
tailed, if properly devised, printed forms are used. They are superior 



PIGEON DESCRIPTION - dorsal 



Follow Sheet No 




o 



Fig. 235. 



-Pigeon description sheet in use in experimental breeding at the Wisconsin 
Agricultural Experiment Station. (Devised by Leon J. Cole.) 

to other less accurate methods of registering data not only because they 
make it easier to set down the data, but because by having items indicated 
on the sheets or cards, it is very easy to see at any time just what data 
remain to be determined. The methods in all cases should be -those 
which are best adapted to the particular conditions which obtain in the 
case in hand. We may summarize in brief the requirements of a good 
system of record keeping by discussing the several features of it. 

The Individual Sheet. — By the individual sheet is meant a sheet 
upon which is recorded vital data for a particular individual. This sheet 
should have places for recording such data as the date of birth of the in- 
dividual, its date of death or disposal, from whom acquired and to whom 
disposed of, and other data of a similar character. This sheet may con- 



604 GENETICS IN RELATION TO AGRICULTURE 

veniently have on its back a pedigree blank for recording all the ancestors 
for at least four generations back. A separate sheet of this kind should 
be made out for at least each breeding individual; individuals which are 
not to be kept for breeding purposes may be noted on other specially 
devised condensed blanks, which give only the necessary essential data 
respecting them. 

The Description Sheet. — The purpose of the description sheet is to 
provide space for notes bearing upon the characteristics of the animal 
in question, short items which may be jotted down from time to time 
whenever they occur to the breeder. This sheet should also bear what- 
ever extended individual descriptions may be necessary. In many cases, 
the use of a printed outline such as that shown in Fig. 235, which is used 
in the investigations of pigeon breeding at the Wisconsin Station, aids 
greatly in making such descriptions definite and detailed without much 
labor. An outline form for instance will aid materially in recording the 
extent and position of black and white areas in Holstein-Friesian and 
other cattle which usually have broken colors. 

The Progeny Sheet. — For recording matings and progeny a special 
sheet is often useful, although it is often possible to provide space for 
this data on the individual sheet. This blank will generally be used in 
the form of a follow sheet to accompariy other sheets of each breeding 
female. Space should be provided for recording dates of service, name 
of 'sire used, date of delivery, sex of offspring, and other vital data of 
this type. There should be a place for recording the disposition of the 
offspring; if added to the breeding herd, a cross-reference should be 
made to its individual sheet. 

The Performance Sheet. — The performance sheet is necessary only 
when the data obtained under this heading are relatively extensive as is 
the case jn milking records of dairy cows or egg records of hens. This 
sheet should be devised in such a fashion as to permit the recording of 
data quickly and accurately. In Fig. 236 is reproduced a summary egg 
sheet such as is used in breeding investigations at the Wisconsin Station. 
It will serve as a type of the kind of sheets which may be used in recording 
data of performance. 

Sheets for Special Purposes. — If the breeder is following out any 
particular type of operations which require special data it should be an 
easy matter to devise sheets which will help him in that matter. As an 
illustration we give in Fig. 237 a reproduction of a sheet used at the 
Wisconsin Station in an investigation of multiple births in cattle. 

General Considerations. — Any system which is adopted should be 
convenient. For that reason a loose leaf system, because it is not bulky 
and offers the maximum freedom in rearrangement and filing, will prob- 
ably prove most satisfactory in practical work. Such systems have 



CONDUCTING BREEDING INVESTIGATIONS 



GOi 



I 





EXPT 






















Breed 
























Year 










9 


Na 












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la 


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Totals 










1 


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3AHDNO 


NOV. 1 Dec. 


Jan. 


Feb 


Mar. 


APR 


May 


June 


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AUQ 


Sept. 


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Fig. 236. — Summary egg sheet in use in experimental breeding investigations at the Wis- 
consin Agricultural Experiment Station. (Devised by Leon J. Cole.) 



ij 



Report only ona pair of twins on each sheet. 

Breed p.cm «.rrr (Sire 




AND NUMBER { 

"^°L3J°''" 1 '"" Reoisteheo) of ( f^^^ 


Aoe OF Dam at Time I HovY Many Times had 1 HAS Dam ever I 
OF Prooucino Tv/ins ( Dam Calved Previously ( had Other Twins 9 1 


Do You Know of Ancestors or Relatives of| 

Sire or Dam that have Produced Twins ? t 


Record twins separately, one under A and the other under B. 
A Sex Name Bcoistwhon Ho. 


Were Health AND Size Normal? 


Was it ever Bred 7 AT What Aoe 7 


Did IT Produce Offsphinq 7 






B Sex Name reqmtiiatioii no. 


Were Health AND Size Normal 7 - 


Was it ever Bred 7 At What Ade 7 






In Case One was a Free-Martin. Describe appearance of Qenital Organs and Udoer 


Record any other points of interest (Compirlton ol horns, color, etc.) on back of this she«t. 

Name and Address 


Date 





Fig. 237. — Printed form used in investigation of multiple births in cattle at the Wisconsin 
Agricultural Experimental Station. (Devised by Leon J. Cole.) 



606 GENETICS IN RELATION TO AGRICULTURE 

been devised for the use of practical breeders. Various aids such as 
different colored sheets for different purposes help to make these systems 
still more convenient. Obviously for the sake of convenience sheets 
should be of the same size so that they may all be filed in the same style 
of binder. The sheets which have been illustrated in this account are 
of size 5 by 8 inches and are very convenient for most purposes. 

Cooperative Breeding. — Most farmers who raise livestock cannot 
afford themselves to keep a good bull for breeding purposes for the few 
cows they have, or still less a stallion for the few mares which they may 
need for their farm labor. Since such a large proportion of stock-raisers 
are in this class it becomes a grave question as to how these farmers may 
be provided with the advantages which accrue from the use of pure-bred 
sires. Any plan which has for its purpose the raising of the general 
average excellence of livestock must take account of these farmers, for 
taken all together they own a very large proportion of the livestock in 
the country, and in the future they will own an increasingly larger pro- 
portion of it. One of the best ways of meeting this difficulty is by 
forming cooperative associations among the farmers themselves for the 
purpose of purchasing pure-bred sires. There is no reason why a given 
section of country should find it necessary to have a different breed of 
horses or cattle or swine on every farm, consequently the first step in the 
formation of such a company should be to agree upon the particular 
breed and type of bull or stallion which should be purchased. Thereafter 
under no circumstances should this decision be changed, but the farmers 
should endeavor to grade their herds up to the highest standard of that 
breed. A definite plan such as this would work an enormous improve- 
ment within a few years in the character of the livestock in a given rural 
district. 

In passing it may be mentioned that it has often been found advisable 
and feasible to lend government aid to the improvement of livestock. 
This has been particularly the case in European countries where long 
decades of breeding have reduced types within a given district to a fair 
degree of uniformity, so that the government might follow a simple 
uniform practice in dealing with a given district. The success which 
such a poHcy may achieve is testified to by the popularity of the Percheron 
and French Coach horses, breeds to which the French government has 
lent considerable official encouragement. These are, however, details to 
be worked out in every section; the important point in every case is to 
follow up thoroughly and consistently for a considerable period of time 
whatever scheme is adopted. 



CHAPTER XXXIX 
CONCLUDING REMARKS 

Although we have discussed a deal of material in this account of 
genetics in relation to animal breeding, it must be apparent to any student 
that we are still woefully lacking in detailed and precise knowledge. 
In fact, as yet we seldom have accurate information with respect to the 
most simple and easily determined matters, such as growth curves in the 
various breeds and races of livestock, comparative production curves, 
and the like, the obtaining of which is largely a matter of routine. 
Such data are not even genetic data, strictly, but they are so necessary 
for the application of genetics to animal breeding that genetics proper 
must almost mark time until they can be obtained. The necessity of 
having accurate standards of judgment obviously need not be debated 
before an intelligent audience. 

Our dearth of detailed knowledge is particularly noticeable, however, 
in the field of genetics proper. It has been said — and there is much 
justification for the statement — that our knowledge of heredity is not 
secure with respect to any character until it has been found possible to 
analyze it and determine the factors which enter into it. If such a cri- 
terion then be applied to our knowledge of heredity in horses and cattle, 
for example, we have little cause to congratulate ourselves upon the 
extent of our accurate knowledge; for in either of these animals the 
number of factors accurately known could be counted on the fingers of 
both hands. It is a far call from such a state of affairs to that obtaining 
in Drosophila ampelophila in which knowledge has been derived concerning 
about 150 factors, many of which have been arranged in a systematic, 
coordinated scheme. Our knowledge is very meagre especially with 
respect to those factors which affect primarily economic characters in 
domestic animals, such as milk production in dairy cows, fertility, vigor, 
and like characters or character-complexes. Here we have a very good 
beginning in Pearl's analysis of winter egg production in domestic 
fowls; but compared with the amount of information yet to be gained 
we see how long is yet the road to be travelled. But this beginning which 
Pearl has given us is very encouraging; it leads us to feel confident that 
our knowledge of accurate details of heredity will be pushed further and 
further. 

607 



608 GENETICS IN RELATION TO AGRICULTURE 

For after all in spite of our present dearth of detailed knowledge of 
heredity in domestic animals there is no real cause for discouragement. 
It is not yet two decades since the rediscovery of Mendel's law of heredity; 
and the most rapid progress has been made within the last five years. 
It is not, therefore, at all strange that we have not yet obtained extended 
data from experimental research; in fact, most of the Mendelian data 
we now have on the larger domestic animals is of the interpretive kind, 
that is, the conclusions have been drawn from records already in existence. 
Experimental research such as has been employed in the study of the 
inheritance of coat color in rodents, has not yet been carried out to 
determine the relations of the various coat colors and patterns in horses 
and cattle; the best thjlt has been found possible thus far is the study of 
herdbook records and breeders' notes. 

The Need of Research. — Students know too well how difficult it is 
to make due allowances for all the variable factors which may enter into 
a given body of data. Consequently, however simple the conditions 
may be, those conclusions which are based on records as crude as those of 
herd books and breeders' notes are subject to a great deal of uncertainty. 
Moreover, it is usually impossible under practical conditions to find 
matings which have been carried out in such a way as to give crucial 
tests of a given hypothesis of factor relations. We have emphasized this 
difficulty in the discussion of Mendelian inheritance in domestic animals, 
pointing out that veiy often alternative interpretations could be made of 
the crude data extant; interpretations which could be very easily sub- 
jected to a crucial test in the case of accurate scientific research. In the 
domestic animals, as in Drosophila, the ideal goal of genetic analysis 
should be 'that which determines accurately the mode of inheritance 
and expression of as many Mendelian factors as is possible. The task 
is difficult, but the increasing knowledge of heredity in lower forms will 
immensely simphfy its execution. 

The time and expense necessary for carrying out studies of heredity 
has often deterred investigators from attacking problems in higher 
animals because the possibility of economic apphcation of the results 
has seemed to be remote or almost certainly nil. But this is not the 
point at issue, as may be clearly seen when the interrelations between 
factors are considered. Accurate determination, for example, of the 
various factors and factor interactions in the heredity of coat color in 
cattle would give a secure and definite basis from which to prosecute other 
investigations more intimately concerned with problems of economic im- 
portance. It is even highly justifiable to commend such investigations, 
because the problem is then first approached in its simplest form. There 
is grave question as to the advisability of plunging pell mell into difficult 
problems before the simpler ones have been solved, were it not for the 



i 



CONCLUDING REMARKS 



609 



fact that simultaneous attack may be made in such investigations both 
against the more obvious and the more obscure questions. 

Since work of this kind requires relatively large funds and consecutive 
attention during many years, it is the kind of research which is eminently 
suited to the facilities provided by agricultural experiment stations. 
In the present state of knowledge in genetic enquiry, investigations in 
heredity to be of value must be planned and directed by carefully trained 
men such as should make up the research staff of experiment stations. 
Undoubtedly as the need for this type of research becomes felt more 
strongly, as it inevitably will when agricultural methods become more 
intensive, special facilities will be provided such as are particularly 
adapted to genetic research. We cannot well apply genetic principles 
to their full value before we have definite genetic knowledge. 

The Service of Genetics. — At the present time genetics can without 
question render an important service to animal breeding, for excellent 




Fig. 2oS. — Liiburatury devoted to genetic research at the University of Illinois. 



as may be the art of the skilled practical breeder it remains a regrettable 
fact that it is neither practised nor known by the great body of practical 
breeders in this country. The great fundamental conception of genetics 
that heredity is the primary guiding hand in determining the character- 
istics of the individual, whether physical or mental, has not become a 
part of the fund of knowledge of the general public. The firmly grounded 
belief of the geneticist that the phenomena of heredity have a definite 
knowable basis are still flouted by the less informed among our practical 
brethren, not only in speech but also in deed, for nothing is more pitiable 
than the blind hope manifested among some of them that something 
good may come out of their hit-or-miss methods of breeding. Superiority 
does not arise from inferiority in animal breeding; planless breeding 
operations are not less deplorable than lack of systematic action in any 
other department of rural activity. 

It is here indeed more than in any other fashion that genetic instruc- 
tion finds its justification. For as more and more men become familiar 
with the laws of heredity and by inference and example broaden the 

39 



610 



GENETICS IN RELATION TO AGRICULTURE 



circle of those who begin to appreciate the significance of those laws, 
it must inevitably follow that general breeding practice will thereby be 
gradually raised. It is not possible for a geneticist, however broad 
his knowledge, to map out rules of procedure in breeding operations such 
that success must inevitably follow their application. Such procedure 
is not to be commended; it is not even scientific, by very nature. For 
intelligent application of the principles of genetics, which is the ideal 
of the scientific animal breeder, presupposes a knowledge of such prin- 




FiG. 239. — Genetics laboratory (for general course) College of Agriculture, University of 

California. 



ciples; the service of the geneticist, therefore, should be to determine 
principles and to indicate insofar as may lie within his power the signifi- 
cance of these principles. 

It is in this direction that the study of genetics is not only advisable 
but needful, for it provides as it were the framework to which the breeder 
may add the necessary empirical elements for the construction of his 
finished plan of procedure. And he will find as he becomes more and 
more familiar with that framework that it is not a mere indifferent 
edifice to which he may attach things here and there as convenience 
dictates, but that it is a coordinated and interrelated structure which 
provides definite places for different kinds of things, so that when 



CONCLUDING REMARKS 611 

they are fitted in their proper phices they tend that much to add to the 
completeness and unity of the whole structure. It is a fortunate breeder 
who is able to approach his problems from such a point of view. 

The Need of Other Knowledge. — Proficiency at any sort of game may 
be gained only by practising the game. No amount of reading and 
study of methods of play will suffice to make a good card player or a 
])illiardist; it is required that the player be able to put the principles 
to effective use if he would achieve any measure of success. It is not 
far different in the practice of animal breeding. Genetics provides 
merely the principles of a game, the effective employment of those 
principles necessitates a thoroughly grounded knowledge of a wide range 
of matters pertaining to the technique of rearing, training, mating, and 
what not of the particular type of animal which is being bred. We 
might say somewhat enigmatically that successful animal breeding 
requires a knowledge both of principles and principals. He who has 
studied genetics has only begun the study of the broader subject of 
animal breeding. Ordinarily it would be a much safer procedure to 
entrust the future of a carefully built-up herd of pure-bred livestock to 
the sympathetic care of the herdsman trained in the old school rather 
than to the most thoroughly trained genetic investigator in the land. 

For after all success in animal breeding depends very largely upon 
the ability of the breeder to build up in his mind an ideal type; and there 
is no more reason or assurance that such a type will arise full-formed in 
the mind of the breeder than that any other good thing may be obtained 
without effort. Here indeed is a rare opportunity for good sound judg- 
ment to work toward a definitely appointed end. For the ideal type of 
the breeder will in a sense be a composite of many types, in determining 
which the particular force of any one factor must be weighed with con- 
summate skill. Thus to take a single illustration, that of the ideal 
type of beef Shorthorn, we may point out some of the types which must 
be welded so to speak into one. There is first the market type of beef 
cattle; broad, deep, built upon the plan of the parallelogram, carrying a 
maximum percentage of high priced cuts, and a minimum percentage 
of ofTal. In the second place we may consider the feeder's type of beef 
cattle. He desires an animal which will lay on flesh rapidly and econom- 
ically. Consequently he looks for a bright and alert, but not overly 
active disposition, and a high degree of functional excellence in the 
digestive system and body in general, such that the animal will consume 
a maximum amount of food and convert it into flesh of the proper quality. 
Perhaps as a slight compensatory allowance here the feeder permits a 
slight increase in volume of digestive and other vital organs with a 
consequent increase in percentage of offal for the sake of more economical 
gains. In the third place we must consider the breeders' type of beef 



612 GENETICS IN RELATION TO AGRICULTURE 

cattle. Here questions of the regularity of breeding, of the type of 
cow best suited for the production of young, of the ability of cows to 
provide sufficient nourishment for their offspring, of adaptability to 
the conditions of climate and to the other environmental features of 
the locality in which they are produced, and many other considerations 
enter in. Finally we have the ideal breed type to consider: the animal 
must possess those characters which distinguish Shorthorns as a breed 
from other beef breeds such as Aberdeen Angus, Galloway, or Hereford 
cattle; and very likely it will be necessary in order for it to meet with 
favor that it display those particular characteristics of the Shorthorn 
breed which mark it as belonging to some favorite family or strain. 
We have seen how difficult it is to deal with Mendelian experiments 
involving differences in five or six definite, allelomorphic pairs of factors; 
how much more difficult must it be to deal with all the variable considera- 
tions which enter into the discussion of the method of constructing an 
ideal beef type of Shorthorn cattle. And yet even in the face of all 
these requirements the results of intelligent, systematic breeding opera- 
tions are surprising in excellence and uniformity of product. When we 
consider this fact we can only become more strongly convinced of the 
definite, knowable operation of the laws of heredity. 

But these factors which enter into the determination of ideal types 
are largely considerations outside the pale of genetics proper. These are 
the matters which must be added to a knowledge of genetics in order to 
complete the equipment which would be at the command of the animal 
breeder. To this knowledge, also, must be added information bearing 
on the technique of managing breeding herds in order to realize the full 
returns which it should be possible to secure. This information will 
include a large and varied range of topics such as the methods of feeding 
breeding stock and of developing young stock, the determination of the 
proper number and use of service animals, methods of coping with disease 
of various kinds; a knowledge of methods and appliances by which the 
greatest possible use may be made of particularly excellent animals, such 
as by artificial insemination, and a thousand and one items to recount 
which would only make this discussion more tedious and uninteresting. 
But these elements are none the less essential to the equipment of the 
successful animal breeder. 

So we come to the end of our account of genetic principles in animal 
breeding, realizing very keenly the limitations in our knowledge, and 
the inadequacy of the principles of genetics alone and unsupported to 
serve as a working equipment for the practical animal breeder. But we 
take a deal of courage and satisfaction out of the fact that a consideration 
of those principles has a proper and important part to play in animal 
breeding, first by the emphasis which it lays upon heredity as a factor 



CONCLUDING REMARKS 613 

in production, and secondly by the firm foundation of coordinated 
principles which it provides as a guide to procedure in breeding operations. 
It is necessary thus to emphasize the importance of heredity as a determin- 
ing factor in production, because of the erroneous ideas which are held 
by the generality regarding the fact of heredity; it is wise to study 
genetics as a guide in breeding practice, because any knowledge which 
is reduced to a basis of known principle or is coordinated with principle 
is that much clearer of comprehension and more assured of intelligent 
application. But with all this the study of genetics has failed of its 
highest purpose, if it has not encouraged in the mind of the student 
the open attitude toward truth and the healthy skepticism of the true 
scientist. For after all problems of animal breeding are problems which 
should be approached in nothing less than a spirit of scientific research, 
problems of infinite complexity but of intense interest as well. 



GLOSSARY 

Aberrant. — Deviating from the normal range of variation of the group in which 
it is placed. 

Aberrations (Chromosome). — Irregularities in chromosome distribution during 
mitosis or meiosis. 

Acquired Character. — A modification of bodily structure or habit which is impressed 
on the organism in the course of the individual life. 

Aleurone. — The protein granules found in the endosperm of ripe seeds. In maize 
the aleurone is confined to a thin layer adjacent to the pericarp. 

Allelomorphs. — Factors occurring in the same locus in homologous chromosomes, 
and for this reason producing "contrasting" or "alternative" characters. 

AUogamous. — Requiring two individuals to accomplish sexual reproduction; also 
applied to plants which are normally cross-fertilized even though capable of self- 
fertilization. 

Amphimixis. — The mingling of hereditary units of two parents in sexual repro- 
duction. 

Antitoxin. — A substance, formed in the body of animals inoculated with certain 
bacteria, which has the power of neutralizing toxins formed by the corresponding 
organisms. 

Atavism. — The appearance of grandparental characters in an individual; con- 
trasted with reversion, which is the appearance of a more distant ancestral character. 

Autosome. — Any other chromosome than the sex-chromosomes. 

Autogamous. — Requiring only one individual to accomplish sexual reproduction; 
normally self-fertilized plants. 

Biometry. — The branch of science dealing with the statistical investigation of 
organic differences. 

Biotype. — A group of individuals all of which have the same genotype. Homo- 
zygous biotypes generally breed true but heterozygous biotypes do not. 

Blastogenic. — Originating in the germ-plasm. 

Bos. — A genus of hollow-horned ruminants having simple horns in both sexes, 
typical of the family Bovida; and the sub-family Bovina;, containing the oxen or 
cattle. 

Breeding. — The art of improving plants and animals, or the experimental in- 
vestigation of genetics by testing, hybridizing and selecting. 

Bud Mutation. — A mutation occurring in the very early history of a bud such that 
a branch is produced which differs genetically from the remainder of the plant. 

Bud Sport. — A branch, flower or fruit which differs genetically from the remainder 
of the plant. 

Calycine Flower. — A peculiar hose-in-hose type of abnormality. 

Capon. — ^A castrated male fowl. 

Castration. — The act of removing the sexual glands. 

Cell. — One of the independent protoplasmic bodies which build up an organic 

tissue. 

615 



616 GENETICS IN RELATION TO AGRICULTURE 

Character. — One of the many details of structure, form, substance or function 
which make up an individual organism. 

Chimera. — A mixture of tissues of different genetic constitution in the same part 
of a plant. 

Chlorophyll. — The vegetable pigment which gives the characteristic color to ordi- 
nary green plants. 

Chromatin. — The most permanent and characteristic constituent of the nucleus; 
so called on account of the readiness with which it becomes colored by certain dyes. 

Chromomeres. — The chromatin granules, which are sometimes arranged like the 
beads on a necklace. 

Chromosome. — A definite aggregation of chromomeres. 

Cleistogamous Flowers. — Those in which development is arrested in the bud but 
which are fertile. The more perfect flowers of the same plant are often nearly or 
quite sterile. This peculiar dimorphism is known to occur in about 60 genera. 

Clone. — A group of individuals produced from a single original individual by 
some process of asexual reproduction, such as division, budding, slipping, grafting, 
parthenogenesis (when unaccompanied by a reduction of the chromosomes), etc. 

Contabescense. — An abortive condition of the stamens and of pollen; of very 
common occurrence in hybrid plants. 

Crossing-over. — Exchange of chromatin material between homologous chromo- 
somes. 

Cross-over Gamete. — A gamete containing one or the other of a pair of homologous 
chromosomes which have interchanged parts by crossing-over. 

Cytology. — The branch of biology which treats of cells, especially of their internal 
structure. 

Cytoplasm. — That portion of the protoplasm of the cell outside the nucleus. 

Dam. — A female parent, referring to mammals; generally with sire as the male 
parent. 

Development. — The complete process of growth of an individual. 

Differentiation. — The process of producing specific parts or substances from a 
general part or substance. 

Dimorphic. — Comprising two distinct forms. 

Dioecious Plants. — Those having the two different sexes on different plants, thus 
insuring cross-fertilization. 

Diploid. — The number of chromosomes normally found in the somatic cells of a 
species; twice the gametic or haploid number. 

Dominant. — Applied to one member of an allelomorphic pair, having the quality 
of manifesting itself wholly or partly to the exclusion of the other member. 

Drosophila. — A genus of fruit flies, D. ampelophila — • the pomace or fruit fly. 

Embryogeny. — Early development of an egg leading to the formation of an 
embryo. 

Embryology. — The science which treats of embryogeny. 

Endosperm. — The substance stored in a seed adjacent to the embryo for its early 
nourishment. 

Epistatic. — Applied to a factor or gene which conditions a certain character when 
present in a genotype which contains a factor or factors in other loci affecting the 
same character; for example, the factor P for purple aleurone in maize is epistatic 
to R, the factor for red aleurone; contrasted with hypostatic. 

Equidae. — The horse family. 

Equus. — The typical genus of the family Equidse. 

Evolution.— The general name for the history of the steps by which any living 
being has derived the morphological and physiological characters which distinguish it. 



GLOSSARY 617 

Factor. — An independently inheritable element of the genotype by the presence of 
which some particular character in the organism is made possible; gene. Sometimes 
referred to as genetic factor or unit factor to avoid possible misinterpretation. 

Fecundity. — The potential reproductive capacity of individuals; the ability to 
produce mature ova or sperm. 

Feral. — Run wild, having escaped from domestication and reverted to a state of 
nature. 

Fertility. — Ability to produce normal, living young; the opposite of sterility. 

Fertilization. — ■'{''he union of male and female sex cells. 

Filly. — A female colt or foal. A young mare. 

Fluctuations (Fluctuating Variations). — The slight differences normally found in 
organisms and attributed either to environmental influences or to recombinations of 
genetic factors. 

Fetus. — An animal embryo in the later stages of development. 

Forehand. — That part of the horse which is before the rider. 

Gamete. — A mature male or female sex cell. 

Gametogenesis. — The process of development of mature sex cells from the pri- 
mordial germ tract. 

Gene. — See factor. 

Genetic Factor. — See factor. 

Genotype. — ^The constitution of an organism with respect to the factors of which 
it is made up; the sum of all the genes of an organism. 

Genus (pi. Genera). — In botany and zoology a classificatory group ranking next 
above the species, containing a group of species (sometimes a single species) possessing 
certain structural characters different from those of any others. 
/"""^^erm. — In contrast with soma, the germ-plasm. 
/ Germ Cells. — Cells specialized for sexual reproduction; the ova and spermatozoa 
/ in animals, the egg cells and pollen grains in plants. 

! Germ-plasm. — -That part of the cell-protoplasm which is the material basis of 

' heredity and is transferred from one generation to another. 
^*^:;^Geo|rbpic. — Turning or inclining toward the earth. 

Graft-hybrid. — A shoot or plant produced by grafting one kind of plant upon 
another and whose characters are intermediate between the characters of the two 
components. 

Graft-symbiont. — ^One member of a graft union. 

Gynandromorph. — An animal in which one side exhibits female characters and the 
other side, male characters. 

Haploid. — The number of chromosomes normally found in the gametes of an indi- 
vidual ; one-half the somatic or diploid number. 

Heifer. — A j^oung cow that has not had a calf. 

Hereditary-complex. — ^The total set of factors in any species conceived as a re- 
action system in which the factors display harmonious interrelations with one another. 

Hermaphrodite. — -Being of both sexes. 

Heterosynapsis. — As applied to the sex-chromosomes, the pairing of an A'- and a 
Y- or a W- and a Z-chromosome. 

Heterotypic Division. — The meiotic or true reduction division by which homolo- 
gous chromosomes are separated into different gametes. ' 

Heterozygosis. — The condition of an organism due to the fact that it is a heterozy- 
gote; the state of being heterozygous; the extent to which an individual is hetero- 
zygous. 

Heterozygote. — A heterozygous individual. 

Heterozygous. — That condition of an individual in which any given genetic factor 



618 GENETICS IN RELATION TO AGRICULTURE 

has been derived from only one of the two generating gametes. Both eggs and sperms 
produced by such an individual are typically of two kinds, half of them containing 
the factor in question, the rest lacking this factor; consequently the offspring of hetero- 
zygous individuals usually consist of a mixture of individuals some of which possess 
the corresponding character while others lack it. 

Homosynapsis. — As applied to the sex-chromosomes, the pairing of two X- or 
two TT-chromosomes. 

Homozygosis. — The state of being homozygous; the extent to which an individual 
is homozygous. 

Homozygote. — A homozygous individual. 

Homozygous. — That condition of an individual in which any given genetic factor 
is doubly present, due usually to the fact that the two gametes which gave rise to 
this individual were alike with respect to the factor in question. Such an individual 
having been formed by the union of like gametes, in turn generally produces gametes of 
only one kind with respect to a given character, thus giving rise to offspring which are, in 
thisregard, like the parents; in other words, homozygous individuals usually breed true. 

Hormone. — A substance secreted or found in some organ or tissue and carried 
thence in the blood to another organ or tissue which it stimulates to functional activity 
or whose functions it inhibits. 

Hybrid. — The offspring of animals or plants of different genotypes, varieties, 
species, or genera. 

Hypertrophy. — An enlargement of a part of the body from excessive growth or 
multiplication of its elements. 

Indigenous. — Native, not exotic. 

Inter se. — Between or among themselves. 

Interference. — Protection from coincident crossing-over of loci on either side of 
the point of crossing-over. 

In utero. — In the uterus or womb. 

Lethal. — Destructive of life. 

Linkage. — That type of inheritance in which the factors tend to remain together 
in the general process of segregation; "gametic coupling" of the older terminology. 

Locus (pi. loci). — A definite point or region in a chromosome at which is located 
a genetic factor or gene. 

Lymantria. — A genus of moths. 

Meiosis. — See reduction or heterotypic division. 

Metabolism. — The sum of the chemical changes within the body, or within any 
single cell of the body, by which the protoplasm is either renewed or changed to per- 
form special functions, or else disorganized and prepared for excretion. 

Mitosis. — Indirect cell division, the characteristic method of multipUcation of 
somatic cells, in which each chromosome is halved longitudinally, one-half passing 
to each daughter cell. 

Monoecious Plants. — Those having both sexes in the same plant. 

Morphology. — The branch of biology concerned with the outer form and internal 
structure (without regard necessarily to the functions) of animals and plants. 

Multiple Allelomorphs. — Factors occupying the same locus of homologous 
chromosomes; the characters conditioned by such factors. 

Mutant. — An individual of a genotypic character differing from that of its parent, 
or those of its parents, and not derived from them by a normal process of segregation 
or by crossing-over. 

Mutation. — The result of a change in genotypic nature independently of normal 
segregation or of crossing-over; strictly an alteration in the fundamental nature of a 
genetic factor. 



GLOSSARY G19 

Non-cross-over Gamete. — A gamete containing a chromosome which has not 
been affected by crossing-over. 

Non-disjunction. — The failure of the two members of a pair of homologous chromo- 
somes to disjoin in the reduction division so that both pass into the same gamete. 

Normal Allelomorphs. — The factors conditioning the characters of the wild or 
normal type of a species as contrasted with the factors which condition mutant 
characters. 

Nucleoplasm. — The protoplasm in the nucleus. 

Nucleus. — The more or less centrally situated cell organ containing the chro- 
matin which has come to be known as the hereditary substance par excellence. 

Ontogeny. — The development of the individual as opposed to phylogeny. 

Ovules. — The macrosporangia of flowering plants; the female sex cells with the 
immediately surrounding parts; the future seeds. 

Paramecium. — -A ciliated protozoan. 

Pedigree. — ^List of ancestors; genealogical tree. 

Peloric Flowers. — Regular flowers borne on plants which normally have irregular 
flowers. 

Pericarp. — In flowering plants the seed vessel or ripened ovary; in maize each 
seed is morphologically a fruit and the seed covering is termed pericarp. 

Petiole. — -Leaf stalk. 

Phenotype. — The sum of the externally obvious characters of an individual or a 
group of individuals. 

Phyletic— Pertaining to ancestral species or groups. 

Phylogeny. — The history of the evolution of a species or group; distinguished 
from ontogeny. 

Phylloxera. — A genus of plant lice; usually of gall-making habits. 

Phylum (pi. Phyla). — A primary division or sub-kingdom of the animal or vege- 
table kingdom. 

Physiology. — The sum of scientific knowledge concerning the functions of living 
things. 

Phytomer. — A plant-part or plant-unit; one of the structures or elements 
which, produced in a series, make up a plant of the higher grade. The ultimate 
similiar parts into which a plant may be analyzed are units consisting of an inter- 
node and a node with its leaves. Each unit may reproduce its like or the entire 
plant. 

Polled. — Hornless. 

Pollen. — The male sex cells of flowering plants. 

Polydactylous. — Having extra fingers or toes. 

Polyphyletic. — Derived from several phyla ; having several different lines of descent. 

Prepotent. — Able to impress individual characteristics upon offspring to a marked 
degree. 

Probable Error. — An arbitrary term used to designate the amount that must be 
added to or subtracted from the observed value to obtain two limiting figures of 
which it may be said that there is an even chance that the true value lies within or 
without these limits. 

Protein. — Complex organic substances containing nitrogen, e. g., albumin or white 
of egg. 

Pure Line. — A group of individuals derived solely by one or more self-fertilizations 
from a common homozygous ancestor. Sometimes erroneously applied to groups of 
individuals beUeved to be genotypically homogeneous (a homozygous biotype or a 
clone) without regard to their method of reproduction. 

Quagga. — A zebra-like animal from South Africa. Named from the sound of its cry. 



620 GENETICS IN RELATION TO AGRICULTURE 

Recombination. — The uniting of parental factors in individuals of the second or 
later generations after a cross. 

Reduction Division. — One of the last two divisions in gametogenesis, when ho- 
mologous chromosomes are dissociated and pass into different gametes; the hetero- 
typic division, meiosis, the mechanism of segregation. 

Recessive. — The opposite of dominant. 

Reciprocal Hybrids. — Hybrids the sexes of w^hose respective parents are reversed. 

Regression. — In biometry, the avarage variation of one variable for a unit varia- 
tion of a correlated variable. 

Reversion. — The appearance of a distantly ancestral character in an individual, 
as the production of purple-flowered sweet peas by crossing two whites. 

Sours. — Abortive horns. 

Segregation. — The process by which genetic factors become dissociated to different 
gametes by the mechanism of the reduction division. 

Sex-chromosome. — The accessory chromosome which has come to be associated 
with one or the other sex, or one member of a pair of morphologically or physiologically 
distinct chromosomes which carry a factor or factors for sex. 

Sex -linked. — Applied to factors located in the sex-chromosomes or to the charac- 
ters conditioned by them. 

Sex -ratio. — -The proportion of males and females in a population. 

Sire. — The male parent of a beast; generally with dam as the female parent. 

Soma. — Body; especially in contrast with the germ or germ-plasm. 

Somatic. — Pertaining to the body as contrasted with germinal which pertains to 
the germ cells. 

Somatic Segregation. — Appearance of genetically diverse tissues in the same indi- 
vidual due usually to mutation in a somatic cell or, possibly, rarely to chromosome 
aberrations, or, in a few questionable cases, to some unknown cause. 

Somatogenic. — Originating in the soma. 

Somatogenesis. — See Development. 

Species. — That which is specialized or differentiated recognizably from anything 
else of the same genus; collectively those individuals which differ specifically from 
all the other members of the genus and which do not differ from one another beyond 
the limits of (actual or assumed) individual diversity. 

Spermatozoon (pi. Spermatozoa). — A mature male sex cell in animals. 

Spindle. — The nuclear division figure. 

Sterility. — Lack of ability to produce normal, living young; the opposite of fertility. 

Stereochemistry. — A branch of chemistry which considers the spatial arrange- 
ment of the atoms composing a molecule. 

Stirp. — As contrasted with body or soma, the germ or germ-plasm. 

Strain. — A group of individuals within a variety which constantly differ in one 
or more characters from the variety type. 

Synapsis. — Non-technically, the conjugation of maternal and paternal chromo- 
somes preceding the reduction division. 

Taxonomy. — The department of science which embodies the principles of classifi- 
cation, especially systematic classification of organisms. 

Teratology. — The phase of morphology concerned with the naming and classifica- 
tion of abnormalities. 

Tetraploid. — Quadruple the haploid or double the diploid number of chromosomes 
characteristic of the species. 

Toxin. — A poison produced in animal tissues. 

Triploid. — Treble the haploid or once and one-half times the diploid number of 
chromosomes characteristic of the species. 



GLOSSARY . 621 

Unit Characters. — Those characters which behave as units in heredity. 

Unit Factor. — See Factor. 

Vaccine. — The modified virus of any specified disease introduced into the body 
by inoculation with a view to prevent or mitigate the disease or to confer immunity. 

Variate. — -A single magnitude determination of a character. 

Variety. — A group of individuals within a species which constantly differ in one 
or more characters from the species type. 

Vegetative Mutation. — A mutation occurring in any somatic cell. 

Zygote. — A fertilized ovum or the product of the conjugation of gametes; hence, 
for brevitj'^, an organism resulting from fertilization. 

Zygotic. — Of or pertaining to a zygote. 

Zygomorphic. — Irregular flowers which are divisible into similar halves in only one 
plane. 



LIST OF LITERATURE CITED 

Explanatory Note. — Inasmuch as a complete bibliography is unnecessary in an 
elementary text the following Ust contains only titles actually referred to directly or 
indirectly in the preparation of this book. In certain cases, viz., the more extensive 
contributors to the literature of genetics, indirect citation is made to lists of works 
given in Castle's "Heredity and Eugenics," designated by Castle, and in Morgan's 
"Mechanism of Mendelian Heredity," designated by Morgan. 

AcKEKMAN, A., 1898. Tierbastarde, Zusammenstellung der bisherigen Beobach- 

tungen. Abhandlungen und Berichte des Vereines fiir Naturkunde in Kassel. I. 

Wirbellose, 40 (1896-7). II. Wirbeltiere, 43 (1897-8). 
Agar, W. E., 1914. Experiments on inheritance in parthenogenesis. Phil. Trans. 

Roy. Soc. Lond., ser. B, vol. 205. 
Allen, G. M., 1914. The heredity of coat color in mice. Proc. Am. Acad. Arts. 

Sci., 40. 

1914a. Pattern development in mammals and birds. A7n. Nat., 48. 
Anderson, W. S., 1914. The inheritance of coat color in horses. Ky. A. E. S. Bull. 

180. 
Arenander, E. O., 1908. Eine Mutation bei der Fjellrasse (Kularasse). Jahrb. f. 

wissensch. u. prakt. Tierzucht. 3. 
Atkinson, G. F., 1914. Segregation of unit characters in the zygote of CEnothera 

with twin and triplet hybrids in the first generation. Science, 39. 
Babcock, E. B., 1913, Studies in Juglans I. Study of a new form of Juglans call- 

fornica Wats. U.C. Pub. Agr. Sci., 2, 1. 

1914. Studies in Juglans II. Further observations on a new variety of Juglans 
californica Wats., and on certain supposed walnut-oak hybrids, U.C. Pub. Agr. 
Sci., 2, 2. 

1915. A new walnut. Jour. Hered., 6. 

1916o. Studies in Juglans III. (1) Further evidence that the oak-like walnut 
originates by mutation. (2) A parallel mutation in Juglans hindsii. U.C. Pub. 
Agr. Sci., 2, 3. 

Babcock, E. B., and Lloyd, F. E., 1917. Somatic segregation — a misleading ex- 
pression. Jour. Hered., 8. 

Bailey, L. H., 1894. A pomological alUance. Yearbook U.S.D.A. 
1897. "Survival of the Unlike," New York. 
1906. "Sketch of the Evolution of our Native Fruits," New York. 

Bailey, L. H., and Gilbert, A. W., 1915. "Plant-breeding," New York. 

Bailey, L. H., and Wyman, A. P., 1896. Sweet peas studies. Ill, Bidl. 189, Cor- 
nell Univ. A.E.S. 

Ballard, W. R., 1916. Methods and problems of pear and apple breeding. Md. 
A.E.S. Bull, 196. 

Barber, M. A., 1907. Heredity in certain microorganisms. Kansas Univ. Sci. 
Bull. 4. 

Barker, B. F. P., 1916. Sweet-pea hybrids. Card. Chron., 60. 

622 



LIST OF LITERATURE CITED 623. 

Bartlbtt, H. H., 1915. Additional evidence of mutation in (Enothera. Hot. Gaz., 

59. 

1915a. The mutations of (Enothera stenomeres. Am. Jour. Bot., 2. 

19156. Mutations en masse. Am. Nat., 49. 

1915c. Mass mutation in (Enothera pratincola. Bot. Gaz., 60. 

1915d. The experimental study of genetic relationships. Am. Jour. Bot., 2. 
Bateson, W., 1894. "Materials for the Study of Variation," London. 

1898. "The Methods and Scope of Genetics," Cambridge. 

1913. The present state of knowledge of color-heredity in mice and rats. Proc. 
Zool. Soc, 2. 

1913o. "Problems of Genetics," Yale Univ. Press. 

19136. "Mendel's Principles of Heredity," third impression, Cambridge (Eng.) 

and New York. 

1914. Address of the President, British Assoc. Adv. Sci., Science, n.s. 40, pp. 
287-302; 319-334. 

1916. Root cuttings chimeras and sports. Jour. Gen., 6. 
Bateson, W., and Pellew, C, 1915. On the genetics of "rogues" among culinary 

peas. Jour. Gen., 5. 
Bateson, W., and Punnett, R. C, 1908. The heredity of sex. Science, 27. 

1911. On the interrelations of genetic factors. Proc. Roy. Soc, 84. 

1911a. The inheritance of the peculiar pigmentation of the silky fowl. Jour. 

Gen., 1. 

19116. On gametic series involving reduplication of certain terms. J our. Gen., 1. 
Bateson, W.; Saunders, E. R.; Punnett, R. C; Hurst, C. C; et al., 1902-1909. 

Reports (I to V) to the Evolution Committee of the Royal Society, London. 

1905. Sweet pea in Roy. Soc. Evol. Rept. II. 
Baur, E., 1907-14. Papers on genetics; bibliography in Castle and Morgan. 

1909. Pfropfbastarde, Periclinalchimaren und Hyperchimaren. Ber. Deutsch. 

Bot. Ges., 27. 

1914. "Einfuhrung in die experimentelle Vererbungslehre," second ed., Berlin. 
Beach, S. A., and Maney, F. J., 1911. Mendelian inheritance in Prunus hybrids. 

Rept. Am. Breed. Assoc, 7. 
Beal, a. C, and Craig, J., 1911. Sweet pea studies I. (a) Introductory. (6) 

Winter-flowering sweet peas. Cornell A.E.S., Bull. 301. 
Beal, A. C, 1912. Sweet Pea Studies II. Winter-flowering sweet peas. Cornell 

A.E.S., Bull. 301. 

1912a. Sweet pea studies III. Culture of the sweet pea. Cornell A. E.S., Bull. 

320. (Contains history of the sweet pea.) 

1914. Sweet pea studies IV. Classification of garden varieties of the sweet pea. 

Cornell A.E.S., Bull. 342. 
Beebe, C. W., 1907. Geographical variations in birds with special reference to the 

effects of humidity. Zoologica, 1. 
Bell, A. G., 1914. Sex determination in sheep. Jour. Hered., 5. 
Bellamy, A. W., 1917. Studies of inheritance and evolution of the Orthoptera. 

IV. Jour. Genet., 7. 
Belling, J., 1913. Third generation of the cross between velvet and Lyon beans. 

Rept. Fla. A.E.S. 

1914. The mode of inheritance of semi-sterility in the offspring of certain 
hybrid plants. Zeit. Abst. Vererb., 12. 

1915. Inheritance of pod pubescence and partial sterility in Stizolobium crosses. 
Rept. Fla. A.E.S. 



624 GENETICS IN RELATION TO AGRICULTURE 

1915a. Inheritance of length of pod in certain crosses, Jour. Agr. Res., 

U.S.D.A., 5. 

19156. Linkage and serai-sterility. Aiti. Nat., 49. 
Benedict, R. C, 1915. Some modern varieties of the Boston fern and their source. 

Joxir. N. Y. Bot. Gard., 16, 189. 

1916. The origin of new v^arieties of Nepholepsis by orthogenetic saltation. 

I. P-ogressive variations. Bull. Torrey Bot. Club, 43, 5. 
BiFFEN, R. H., 1905. Mendel's laws of inheritance and wheat breeding. Jour. 

Agric. Set. Cambridge, 1. 
Blair, F. J., 1916. Development and locahzation of truck crops in the United States. 

Yearbook U.S.D.A. 
Blake, M. A., 1913. Individuality in rose plants. Proc. Soc. Hort. Sci. 
Blakeslee, a. F., and Warner, D. E., 1915. Fancy points vs. heredity. Jour. 

Hered., 6. 

1915a. Correlation between egg laying and yellow pigment in the domestic fowl. 

Am. Nat., 49. 
Blaringhem, L., 1913. "Le Perfectionment des Plantes," Paris. 
Blinn, p. K., 1908. Breeding a leaf blight resistant cantaloupe. Colo. A.E.S., Bxdl. 

126. 
BoLLEY, H. L., 1912. Importance of maintaining a constant elimination factor in 

association with a nutrition factor in plant breeding. Proc. Am. Breed. Assoc, 8. 
Bond, C. J., 1912. On heterochromia iridis in man and animals from the genetic 

point of view. Jour. Gen., 2. 
Boring, Alice M., and Pearl, Raymond, 1914. The odd chromosome in the sper- 
matogenesis of the domestic chicken. Joiir. Exp. Zool., 16. 
Boshnakian, S., 1916. Breeding Nephrolepis ferns. Jour. Hered., 7 
BovERi, Th., 1907-14. Important papers on cytology; bibliography in Morgan. 
Bowater, W., 1914. Heredity of melanism in Lepidoptera. Jour. Gen., 3. 
BoYD, M. M., 1906. Breeding of polled Herefords. Repts. Am. Breed. Assoc. II. 
Bridges, C. B., 1913. Partial sex linkage in the pigeon. Sci., 37. 

1913o. Non-disjunction of the sex chromosomes of Drosophila. Jour. Exp. 

Zool, 15. 

1914. Chromosome hypothesis of linkage applied to cases in sweet peas and 
primula. Am. Nat., 48. 

1914a. Direct proof through non-disjunction that the sex-linked genes of Droso- 
phila are borne by the X-chromosome. Sci. n. s., 40. 

1915. A linkage variation in Drosophila. Jour. Exp. Zool., 19. 

1916. Non-distribution as proof of the chromosome theory of heredity. Gen- 
etics, I. 

1917. An intrinsic difficulty for the variable force hypothesis of crossing-over. 
Am. Nat., 51. 

1917a. Deficiency. Genetics, 2. 

Bridges, C. B., and Sturtevant, A. H., 1914. A new gene in the second chromo- 
some of Drosophila and some considerations on differential viability. Biol., Bull. 
26. 

Briem, H., 1911. Seed roots in beet raising. Abstr. in Mo. Bull. Agr. Intell., 2. 

Brooks, C, 1914. Bottom-end rot of tomatoes. Phytopathology, 4. 

Buder, J., 1915. Chimaren und Pfropfmischlinge. Die Naturwissenschaften, Bd. 3, 
s. 6, 23 and 33. 

Bull, C. B., 1908. Corn breeding in Minnesota. Minn. A.E.S., Bull. 107. 

Camerarius, (Camerer) R. J., 1694. "Epistola de sexu Plantarum." 

Campbell, D. H., 1911. Tlie nature of graft-hybrids. Am. Nat., 45. 



LIST OF LITERATURE CITED 625 

Castle, W. E., 1903-16. Papers by Castle el al; bibliography in Castle and Morgan. 

1916. "Heredity and Eugenics," Harvard Univ. Press. 

1916a. Further studies on piebald rats and selection with observations on 

gametic coupling. Carnegie Pub. 241, part III. 
Carleton, M. a., 1916. "The Small Grains," New York. 
Carrier, L., 1910. Preventing cross-pollination of corn by means of muslin screens. 

Paper read before Amer. Soc, Agron. 

1913. The immediate effect on yield of crossing strains of com. Va. A.E.S., 
Bull. 202. 

Carrol, W. E., 1917. Selecting dairy bulls by performance. Utah A.E.S. Bui, 153. 

Clark, C. F., 1910. Variation and correlation in timothy. Cornell Univ. A.E.S. , 
Bull. 279. 

Clark, J. Allen, 1916. Improvement of Ghirka spring wheat in yield and quality. 
U.S.D.A., Bull. 450. 

Clausen, R. E., and Goodspeed, T. H., 1916. Hereditary reaction-system rela- 
tions — an extension of Mendelian concepts. Proc. Nat. Acad. Sci., 2. 

Clausen, R. E., 1915. Ettersburg strawberries. Jour. Hered., 6 

Cockerell, T. D. a., 1912. The red sunflower. Pop. Sci. Mo., 80. 

1917. Adult characters in sunflower seedlings. Jour. Hered., 8. 
1917a. Somatic mutations in sunflowers. Jour. Hered., 8. 

CoiT, J. E., 1910. The relation of asexual or bud mutation to the decadence of 
California citrus orchards. Proc. Calif. State Fruit Growers Convention (Pomona). 

1915. "Citrus Fruits." New York. 

1917. Recorded trees vs. pedigreed trees. Calif. Cultivator, Los Angeles. 
Cole, L. J., 1912. A case of sex-linked inheritance in the domestic pigeon. Science, 
n. s., 36. 

1914. Studies on inheritance in pigeons. R.I. A.E.S. Bull. 158. 

Cole, L. J., and Wright, W. H., 1916. Application of the pure-line concept to 

bacteria. Jour. Infec. Diseases, 19. 
Collins, G. N., 1909. The importance of broad breeding in com. U.S.D.A. Bur. 

Pit. Ind., Bull. 141, part 4. 

1910. The value of first generation hybrids in corn. U.S.D.A. Bur. Pit. Ind., 
Bull. 191. 

1910a. Increased yields of corn from hybrid seed. Yearbook, U.S.D.A. 

1911. Inheritance of waxy endosperm in hybrids of Chinese maize. Proc. Ath 
Intern. Conf. Genetique, Paris. 

1912. Gametic coupling as a cause of correlations. Am. Nat., 46. 

1913. Heredity of a maize variation. U.S.D.A. Bur. Pit. Ind. Bull, 272. 

1914. Nature of Mendelian units. Jour. Hered., 5. 

1914a. A more accurate method of comparing first-generation maize hybrids 
with their parents. Jour. Agr. Research, 3, 

1917. Hybrids of Zea raniosa and Z. tunicaia. Jour. Agr. Research U.S.D.A. 9. 
Collins, G. N., and Kempton, J. H., 1913. Effects of cross pollination on the size' 
of seed in maize. Cir. 124, U. S. D. A. Bur. Pit. Ind. 

1914. Inheritance of endosperm texture in sweet and waxy hybrids of maize. 
Am. Nat, 48. 

1916. Patrogenesis. Jour. Hered., 7. 

Conferences on Genetics, Report 3rd Intern., London, 1907. Rept. 4th Intern., Paris, 

1911. 
CoNKLiN, E. G., 1915. "Heredity and Environment," Princeton Univ. Press. 
Cook, O. F., 1904. The vegetative vigor of hybrids and mutations. Proc. Biol. 
Soc. Wash., 17. 

40 



626 GENETICS IN RELATION TO AGRICULTURE 

1908. Reappearance of a primitive character in cotton hybrids. Cir. 18, 
U.S.D.A., Bur. Pit. Ind. 

1909. The superiority of line breeding over narrow breeding. U.S.D.A., Bur. 
Pit. Ind., Bull. 146. 

1909c. Suppressed and intensified characters in cotton hybrids. Bull. 147, ibid. 

19096. Local adjustment of cotton varieties. U.S.D.A., Bur. Pit. Ind., Bull. 159. 

1911. Dimorphic branches in tropical crop plants. U.S.D.A., Bur. Pit. Ind., 

Bull. 198. 

1911a. Dimorphic leaves of cotton and allied plants in relation to heredity. 

U.S.D.A., Bur. Pit. Ind., Bull. 221. 

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628 GENETICS IN RELATION TO AGRICULTURE 

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630 GENETICS IN RELATION TO AGRICULTURE 

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632 GENETICS IN RELATION TO AGRICULTURE 

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634 GENETICS IN RELATION TO AGRICULTURE 

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41 



642 GENETICS IN RELATION TO AGRICULTURE 

Stockard, C. R., 1913. The effect on the offspring of intoxicating the male parent 

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Stockard, C. R., and Papanicolau, G., 1916. A further analysis of the hereditary 

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Stockberqer, W. W., 1916. Relative precision of formulie for calculating normal 

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Stok, J. E. VANDER, 1910. Account of the results obtained by crossing of Zea mays 

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Stuckey, H. p., 1916. Transmission of resistance and susceptibility of blossom end 

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1913. The linear arrangement of six sex-linked factors in Drosophila, etc. 
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1913o. The Himalayan rabbit case, with some considerations on multiple 

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INDEX 



Aberrant, form and chromosome number, 263 

forms in hybrid races of Antirrhinum, 248 

organisms, 276 
Aberrations, chromosome, 263, 274, 285 
Abnormal abdomen in Drosophila, 134 
Abraxas, chromosomes in, 208 

grossulariata X lacticolor, 206, 207 

sex-inheritance in, 205 
Abutilon, striatum, 382 

thomsoni, 382 
Accidental errors, 425 
Ackermann, species hybrids, 228 
Acquired characters, argument against, 484 

belief in inheritance, 482, 483, 494 

definition of, 480 

evidence against, 483, 485, 494 
of inheritance of, 489 

in animal breeding, 480 

parallel induction, 492 

supposed cases, 489 

use and disuse, 491 
Adaptation and factor mutations, 286 

and natural selection, 439 
(Edes calopus, 528 

iEgilops triticoides, hybrid origin, 438 
Agar, mutation, 286 
Agouti, factors in Cavia, 225 
Agriculture antedates plant improvement, 287 

factor mutations caused, 267 

importance of mutations, 263 
Albino plants, 131 
Albinic beetles, 258 
Albinism in plants, 248 
Alcohol, effect of, on animals, 493 
Alder. Fi hybrids in, 365 

Aleurone color inheritance in maize, 83, 105, 163 
Alfalfa, clonal selection in, 298 

disease-resistant strain, 417 

extension of culture, 296 

flowers, 349 

Grimm, 296 

hybrids, 296 

hybridization of, 348 
Algebraic method of computing Fi results, 97 
Alkali resistance, 401 

Alkaloids, transmission from stock to scion, 384 
Allelomorph, 72 

multiple, 155, 210 

normal, usually dominant to a mutant factor, 
145 

of white-eye factor, 74 



Allelomorphs, multiple, 155, 210 
quadruple, 210, 225 
septuple, 156 
triple, 225 
Allen, grape hybridizer, 290 
Allogamous organisms, 16 

have no pure lines, 256 
Almond and peach grafting, 383 

-peach graft-hybrid, 378 
Altenburg and Muller, truncate wings in Droso- 
phila, 168 
Alterations in genetic factors, 263 
Alternative expression vs. transmission, 275 
Althea cannabina X, A. officinalis, 230 
Amateur plant breeder, 312 
Amelioration of plants and animals, 11 
American Breeders' Association, 300 
American Breeders' Magazine, 300 
American Genetic Association, 301 
Amphimixis, 3, 453 
Amygdalus communis-persica graft-hybrid, 378 

platycarpa X persica, 297 
Analysis of coat color inheritance, 465, 470 
Ancestral inheritance, law of, 5 
Ancestry, diversity of, in domestic animals, 460 
Andalusian *owls, see Blue Andalusian Fowls. 
Anderson, Mendelism in horses, 466 
Animal breeder, equipment of, 450 

breeding, acquired characters in, 480 
and disease, 522 
and hybridization, 508 
crossbreeding, 514 
foundation stock, 502 
history of, 443 
industry, 445 
methods of, 577 
practice, 447 
problems of, 447 
role of selection in, 494 
selection problem in, 495 
Animals, domestication of, 443 
fertility in, 551 
genetic data on, 608 
improvement of, 444 
mutation in, 462 
sex in, 536 
sterility in, 555 
variation in domestic, 453 
Anthothrips verbasci, sex-determination in, 211 
Antirrhinum, aberrant forms in hybrid races, 248 
factors, 223 

intermediate expression, 147 
linkage, 125 



649 



650 



INDEX 



Antirrhinum, majus X moUe, 220 

majus, 90, 133 

species hybrids, 227, 235 
Antitoxins in plants, 408 
Aphis maidiradicis, maidis, 410 

-resistance in stone fruits, 409 
in teosinte, 410 
Apple, Baldwin, 393 

Ben Davis, 394 

breeding, 295, 297 

bud selection, 393 
sports in, 393 

chimeras, 381 

collections, 299 

hybridization, 311 

uniformity, 393 

yield, 393 
Aquilegia hybridized, 438 

multiple allelomorphs in, 160 
Arctia caja, 492 

Arenander, mutation in cattle, 464 
Arnold, apple breeding, 295 
Art of breeding, 447 
Artificial insemination, 552 
Asparagus, 318 

alkali resistance, 401 

heterosis in, 364 
Ass, chromosome number in, 519 

hybrids, 516 
Autogamous organisms, 16 
Autogenetic variations, 20 
Autosomes, 58, 65 
Ayrshire breed, 454 



Babcock and Lloyd, somatic segregation, 275 
Back-cross, 73 

Bacteria contain chromatin, 264 
Bags for hybridization, 344 
Bailey, L. H., bud mutation, 366 
selection, 298 

seed and plant introduction, 299 

"Survival of the Unlike," 288 
Bailey and Wyman, sweet pea, 305 
Bake well, animal breeder, 500, 581 
Banana, dimorphic branches, 275 
Barber, genetic theory, 27 
Bar eye in Drosophila, 149, 202 
Barley, 317 

autogamous, 257 

little natural crossing, 318 

mutations in, 369 

pure lines, 293, 340 

winter-resistant, 340 
Barrenness in Duchess cows, 556 
Bartlett, CEnothera investigations, 283 
Bateson, comb types in fowls, 164 

complementary factors, 140 

contabescence, 234 

earwig, 48 

fowl, sex-linkage, 209 

genetics, meaning of, 1 

linkage in peas, 125 



Bateson, multiple recessive factors, 140 

presence and absence hypothesis, 153 

polled cattle, 463 

Bomiatic segregation, 274 

sweet pea, 306, 308 
Bateson and Punnett, Sheep, 549 
Baur, Antirrhinum, 147, 220, 223, 227, 235 

chimeras, 378, 379, 380 

factors, manifold effects of, 133 

linkage in snap dragon, 125 

natural chimera, 378 

Primula experiments, 26 

Solanum Darwinianum, 377 

snap dragon trihybrid, 90 

stock and scion, 383, 384 

variation classified, 15, 263 

variegation, 381 
Beach and Maney, aphis-resistant plums, 297 

hybrid fruits, 409 
Beal, sweet peas, 304, 306, 307, 310 
Bean, 317 

Bush Lima, origin, 177 

Florida Velvet, valuable mutations in, 
367 

multiple allelomorphs in, 160 

mutations in, 367 

Princess, 250, 318 
Beans, crossing in, 318 

effect of grafting, 384 

pure lines in, 340 

tall and dwarf, 174, 175 
Becker, Boston fern, 312 
Bees, bumble, cross beans, 318 
Beets, alkali resistance, 401 

remnant system, 335 
Behavior of graft-hybrids, 382 
Beliefs of practical breeders, 564 
Bell, marking individuals, 602 
Benedict, Boston fern, 312 

bud mutations, 315 
Berckman's peach, 297 
Bermuda grass, clonal selection of, 298 
Berries, hybrid, 297 
Berthollet, species hybrids, 230 
Bibos frontahs X Bos taurus, 236 

gaurus X Bos taurus, 236 

grunniens X Bos taurus, 236 

sondaicus X Bos indicus, 236 
BiflSn, Puccinia glumarum, 411, 412 

wheat, rust-resistant, 294, 295 
Biologic strains of fungi, 415 
Biology and acquired characters, 480 

subdivisions of, 4 
Biometrical school, 32, 439 

study, requirements of, 36 

terms, 37 
Biometry, 5, 6, 32 
Biotypes in maize, 326 

in mixed populations, 260 
Birds, hybrid, 521, 543 

sex-determination in, 539 
sex-linked inheritance, 209 
song in male, etc., 214 
sterility in hybrid, 558 



INDEX 



651 



Bison americanus, 533 

americanus X Bos taurus, 230, 520, 533, 542 

disease immune, 529 
Bittersweet, graftago of, 374 
Bizarria, 378 
Blair, truck crops, 441 
Blake, bud selection, 398 
Blakeslee, egg production, 507 
Blakeslee and Warner, poultry studies, 507 
Blastogenic characters, 480 
Blending inheritance recognized by Castle, 189 
Blood eye color in Drosophila, 156 
Blount, grain rust, 411 

hybridization of plants, 294 
Blue Andalusian fowls, 152 
Bolley, flax, 417 
Bos taurus hybrids, 236 

sterility of, 557 
Boshnakian, Boston fern, 313, 314 

Nephrolepsis, 316 
Boston fern, dwarfing, 315 

increase in leaf division, 315 
in ruffling, 315 

origin of varieties in, 312, 391 

progressive mutations in, 315 
Boyd, polled Herefords, 463 
Brassica campestris, 19 
Breeder, animal, equipment of, 450 
Breeders, beliefs of, 564 
Breeding, and chromosome numbers, 342 

animal and plant compared, 447 

art of, 447 

conservative and constructive, 440 

cooperative, 606 

defined, 11 

disease-resistant plants, 295, 400 

for immunity, 529 

from the best, 493 

genetic investigations needed, 610 

investigations, 591 

maize, 325 

methods and selection, 500 

of crop plants begun, 288 

operations, planning, 440 

other knowledge needed. Oil 

pedigree, 580 

plant, 287, 291, 299 

practical genetics and, 449 

problems of animal, 447 

projects, 440 

systems, 580 

value and modifiability, 456 

wheat, 336 
Breeds, dual purpose, 515 

knowledge of, 451 

utilization of older, 448 
Brentana, Phasianidae, 521 
Bridges, chromosomes, sex, 555 

crossing-over dependent on age, 120 

"deficiency," 121, 155 

eosin eye color, modifiers, 192 

interference and linkage, 124 

lethal factor, 546 

multiple allelomorphs in eye color, 161 



Bridges, non-disjunction, 198-205 

sterility in Drosophila, 234 
Bricm, sugar beet, 340 
"Broadbreeding" advocated, 334 

begun in Germany, 292 
Bryonia, sex-inheritance, 196 
Bryonia dioica, sex-inheritance, 212 
Bryophyllum calycinum, hormones, 276 
Bud mutations, 385 

in Boston fern, 315, 386 
in cherry, 383 
in citrus, 392 
in roses, 311 
long known, 366 
progressive and digressive, 315 
selection, 298, 385, 386 
effective in Coleus, 390 
in apples, 393 
in citrus, 391 
in horticulture, 391 
in potato, 394 
limitations of, 398 
sports, 270 
cause, 273 
in apple, 393 
in orange, 386 
in potato, 394 
occurrence, 272, 311, 386 
variations, 298, 385, 391 
Budd, seed and plant introduction, 299 
Buder, Cratsegomespili, 379 
graft-hybrids, 374, 377 
graft-hybrids list, 378 
reciprocal grafting, 382 
Buds, adventitious and chimeras, 374 
Buff eye color in Drosophila, 150 
Bull, Concord grape, 290 
plant breeding, 437 
tree fruits, 297 
Bumble bees, crossing beans, 318 
Burbank, composite hybridization, 298 
species crossing, 297 
walnuts, 364, 365 
Bursa bursa-pastoris X B. heegeri, 137 

duplicate factors in, 130 
Butter fat production in mother and daughters, 
458 
records compared, 455 



Cabbage, disease-resistant strains, 418 
Cacao, dimorphic branches, 275 
Calycine flower in Nicotiana, 218 
Canadian Seed Growers' Association, 301 
Cannabis sativa, 270 
Cantaloupe, 317 

disease-resistant, 415 
Carbohydrates, specificity of, 260 
Carleton, experimental error, 430 

grain rust, 413 

Marquis wheat, 294 

small grains, 290 



652 



INDEX 



Cammerer (Comerarius), "Epistola de Sexu 

Plantarum," 437 
Carnation, bud sports in, 398 

species and varieties, 302 
Carrier, effect of crossing, 360 
Carri§re, bud mutations, 366 
Carrot, selection, 290, 292 
Castanea americana, mollissima, 406 

primula X crenata, 410 
Castle, blending inheritance, 189 

color in horses, 466, 468 

crossing-over, 576 

dihybridism in guinea-pig, 85 

effects of selection, 334 

factors variable, 192 

fruit fly, 553 

guinea-pig, 224, 553 

heredity defined, 3 

hooded rats, 186, 188, 192, 454 

Mendelian inheritance, 188 

mutant rats, 190 

selection studies in rats, 190, 191 

selection, 258 
in corn, 328 

size inheritance, 188 

sterility, 554 
Castle and Phillips, acquired characters, 486 

hooded rats, 187 

ovarian transplantation, 485, 486, 487, 488 
Castle and Wright, guinea-pigs, 558 

linkage in rats, 125 
Castration, effects of, in animals, 548 

in wheat, 348 

of hermaphrodite flowers, 343 
Cat, sex-linked characters in, 538 

sterility in tortoise-shell, 555 
Cattalo, 535 

Cattle, Aberdeen Angus, 461, 473, 514, 515, 534, 
572 

Alderney, 490 

Ayrshire, 454, 471, 475, 572 

bison hybrids, 533, 542 

Blue gray, 473 

breeding, 500 

crossed with the yak, gaval, gaur, zebu and 
bison, 520 

Dexter-Kerry, 527 

diversity of ancestry, 460, 461 

Durham, 463 

Dutch Belted, 475 

face pattern, 454, 474, 534 

Galloway, 473, 514 

Guernsey, 510, 574 

grading southern range, 529 

Hereford, 454, 463, 508, 529, 534, 584 

Holstein Friesian, 445, 454, 458, 463, 471, 
510, 572, 594 

hornlessness dominant, 267 

Jersey, 511 
Mendelism in, 471 
mutations in, 463 
niata, 276 
Old Sussex, 455 

Park, 471 



Cattle, polled, 463 

sex-determination in, 536 

Shorthorn, 461, 463, 471, 472, 501, 514, 529 
555, 572, 584, 596, 598, 611 

Shorthorn X Hereford, 475 

Zebu hybrids, 520, 531 
Cavia porcellus X rufesceus, 223, 235, 575 
Cavy, factors, 223 

hybrids, 223, 235, 542 
Cell division, 59, 61 

mechanism, 274 
Cells, 57, 58 

in graft-hybrids, 374, 376, 378, 380 
Centgener method, 293 
Cereals, species and varieties, 302 
Characters, 71, 72 

acquired, 480 

in domestic animals, 454 

secondary sexual, 214, 548 

sex-hnked, 197, 214 

"thrust" and "achieved," 481 
Chase, orange and lemon, 391 
Checkerboard, dihybrid, 88 

method of Punnett, 98, 99 

trihybrid, 92 
Cherry, breeding, 297 

eye color in Drosophila, 156 

stock-scion effect, 383 
Chestnut, bark disease, 405 

Chinese, resistant to Endothia, 408 

hybrids, disease-resistant, 410 
Chimera, defined, 270 

in tomato, 272 

natural, 378 
Chimeras, 270, 374 

cells in, 377, 380 

color, 381 

due to factor mutations, 271 

hyperchimeras, 375 

in citrus, 378, 381 

in orange, 375 

natural, 272, 381 

periclinal, 375, 380 

sectorial, 375, 380 

structure, 381 

tissues in, 377, 380 

tomato-nightshade, 374 
Chinquapin-chestnut hybrids, 410 
Chromatin, 59, 63, 67 

in bacteria, 264 

interchange between homologous chromo- 
somes, 63, 65, 67, 108 

the only permanent constituent of nucleus, 
266 

volume in two forms of primula, 264 
Chromomeres, 59, 67, 73 
Chromosome, aberrations, 263, 274, 285, 366 

constitution, non-disjunction and sex, 205 

dimensions and phylogeny, 264 

interpretation of dihybrid, 83 
of linkage, 108 
of monohybrid, 73 
of multihybrid, 95 
of trihybrid, 91 



I 



INDEX 



653 



Chromosome, mechanism, 57, 59, 61, 65, 105 
number and breeding, 342 
and selection, 257 
and specification, 263 
in horse and ass, 519, 537 
relations in sex-inheritance, 196, 205 
theory of heredity and linkage, 104, 117 
Chromosomes, 57, 58, 59 

aggregates of chromatic material, 59 

and fertility, 555 

behavior in non-disjunction, 199 

combinations of, in germ cells, 04, 65 

deviations in number, 263 

diploid and haploid numbers, 65, 06 

independent distribution of, 63 

individuality of, 59 

in graft-hybrids, 374 

in male and female honey bees, 210 

in related forms, 204 

interchange of chromatin between, 63, 65, 67 

maternal and paternal, 60, 64, 65 

number in Abraxas, 208 

in Drosophila, maize, wheat, man and 

tobacco, 57, 184 
in Nicotiana, 184 
in wheat and rye, 238 
occurrence in pairs, 58 
phylogeny of, 204 
Chrysanthemum, bud sports in, 398 
collections, 299 
species and varieties, 302 
Citrous fruits, chimeras, 381 
bud selection, 391 
undesirable sports, 391 
Citrullus vulgaris, 414 
Citrus breeding, 297, 300 
bud mutations in, 392 
chimeras, 375, 37§, 381 
collection, 300 
group, research on, 300 
pollen, 300 
Clark, Ghirka wheat, 372 
Classifications of variations, 15 
Clonal diversity, 302 

multiplication, 296, 311 
selection, 291, 293 
selections, propagation, 298 
variation, 391 
Clone, 257 

propagation of, 298 
Close-pollinated plants, selection in, 336 
Clover, at Svalof, 293 
red, 317 

remnant system, 335 
Club wings in Drosophila, 134, 135 
Cockerell, red sunflower, 270 
CoeflBcient of correlation, 52 

as index of inheritance, 459 
calculation of, 53 
interpretation of, 54 
of inbreeding, 598 
of relationship, 000 
of variability, 44 
in Fa families, 185 



Coffee, dimorphic branches, 275 
Coit, Bizarria, 378 

orange chimeras, 375 

"recorded" stock, 394 

undesirable sports, 391 

Washington navel orange, 392 
Cole, Clydesdale, 467 

record sheet, 003, 605 

reversion in pigeons, 172 
Colcus, biumci, 389 

bud mutations, 388, 390 
selection in, 386, 389, 390 
sports, 386 

color patterns, 387 

composite crossing in, 386 

fluctuating variation, 380, 390 

history of garden, 380, 389 

lacinate-leaved, 380, 389 

pedigree method, 390 

pigmentation, 389 

seed progeny, 389 

somatic factor mutations, 388 
Collections of plant-breeding material, 299 
Ceilings, breeder, 501 
Collins, comparing yields, 350, 357 

corn, purple X white, 105, 358 
•increase in yield by hybrids, 354, 359 

narrow breeding, 334 

seed corn production, 301 
Collins and Kempton, effects of crossing, 300, 301 
Color-blindness in man, 197 

-chimeras, 381 
Comb characters in fowls, 104 
Combinations, 15, 99 

of factors in crossing-over, 108 

of gametes in Mendelian experiments, 100 

sesqui-hybrid, 235, 237, 244, 248 
Commelin, sweet pea, 303 
Cook, cotton, 295 

dimorphic branches and leaves, 275, 276 

"new creations," 440, 441 
Cooper, plant breeding, 437 

selection, value of, 288 
Complementary factors, 140 
Composite crossing, fruits and flowers, 297 

grains, 296 

in Coleus, 380 

roses, 311 
Composition of plant populations, 317, 321 

of self-fertilized populations, 320 
Concept of factor mutation, 207 

of reaction system relations, 238, 242, 248 
Contabescence, factor for, in sweet peas, 234 
Contagious abortion in cattle, 552 
Contamination, allelomorphie, assumed, 192 

of factor, hypothesis of, 190 
Continuous variations, 18 
Cooperative breeding, 606 
Corbett, seed impurity, 428 
Corn, see Maize. 
Correlation, 49 

and linkage, 127 

and modifiability, 459 

and selection, 49, 506 



654 



INDEX 



Correlation, between economic characters, 442 

between mothers and daughters, 457, 458 

classified, 55 

coefficient of, 52, 430, 458 

in plot yields, 430 

table, 49, 51 

interpretation of, 50, 52 
method of tallying, 50 
Correns, Bryonia alba, 212 

effects of crossing, 360 

Mirabilis, variegated, 273 

rediscovery of Mendel's principles, 68 

starchiness in corn, 79 

somatic variation, 272 
Cotton, 317 

collecting stock seed, 341 

dimorphic branches and leaves, 275 

hybridization, 294 

leaf factor, 178, 179 

local adjustment of varieties, 441 

mass selection in, 292 

natural crossing in, 318 

pedigree breeding, 294 

plant-to-row tests, 433 

wilt-resistant selections, 417 
CoupHng factor, 104, 126 

Cowpea, wilt immune variety, 416 • 

Crampton, grafted insects, 382 
Crat£egus-Mespilus graft-hybrids, 378, 379 

monogyna, 378 
Creating a gray breed of horses, 470 

new varieties, methods of, 302 

populations at Svalof, 352 

rust-resistant wheat, 411 
Creation of bantam varieties of fowls, 500 

characters, 342 

disease-resistant plants, 400 

varieties of the rose, 310 

wilt-resistant watermelons, 414 
Cretinism, 526 

Crop plants, breeding begun, 288 
improved, 440, 441 

earlier breeders of, 289 

species and varieties, 302 
Cross-breeding for rust-resistant wheat, 411 

livestock, 514, 584 
Crossing, composite, 296 

effect on plant populations, 321 

immediate effect of, 360 

results of promiscuous, 343 
Crossing-over, 108, 109 

and non-disjunction, 202, 204 

and prepotency, 576 

between twelve sex-linked factors, 121, 123 

double, 119 

for white, miniature and bar in Drosophila, 
118-120 

in male Drosophila, no, 114, 209 

in male rat, 576 

in sex-heterozygote, no, 209 

in silk-worm, 210 

mode of interchange in, 121, 123 

percentage between loci, an indication of 
distance between factors, 117 



Crossing-over, triple, 121, 123 

Cross-over combinations of factors, 108, 119 

gametes, 119 
Cucumber, 317 

increased yield in Fi, 362 
Cuenot, multiple allelomorphism, 155 
Culley, inbreeding, 581 
Cumulative factors, 140 
Cuvier, fertility, 438 
Cydonia-Pyrus graft-hybrid, 378 
Cytisus Adami, 378, 382 

-Laburnum graft-hybrid, 378 

purpureus, 378 
Cytological investigations in insects, 210 

studies in ffinothera, 285 
Cytology, method of, in genetic research, 8 

D 

Dahlia chimeras, 381 

species and varieties, 302 
Dairy cows, comparison of records, 455 

marvellous records of, 454 

obtaining records, 592 

Tilly Alcartra, 594, 597 
Daniel, stock and scion, 383, 384 
Daniel and Delpon, graft-hybrid, 378 
Daniel and Elder, stock and scion, 384 
Darbishire, dominance, 145 

pea crosses, 294 
Darwin, aberrant individuals, 276 

acquired characters, 6, 480 

adaptation, 438, 439 

Ancon sheep, 462 

"Animals and Plants under Domestication, 
276, 453 

cattle, 471 

dimorphism, 275 

evolution theory, 5 

fertility, 552 

hyacinth, 287 

mule, 517 

natural selection, 14, 276 

on Shirreff (cereals), 289 

"Origin of Species," 276, 437 

plant improvement, 287 

quagga, 556 

reversion in pigeons, 171 

selection, 250 

sexual selection, 215 

theory of pangens, 489 

use and disuse, 491 

variations transmitted, 6 

variety crosses, 231 

vigor, 553 

zebra-ass hybrid, 520 
Darwinian theory of evolution, 439 

fertiUty, 552 

selection, 495 
Date palm, 318 

artificial pollination, 287 
Dates, research on, 300 
Datura, 438 

vigor in species hybrids, 230 



INDEX 



655 



Davenport, C. B., "bleeders," 524 

Mendelism in sheep, 476 

ovarian regeneration, 487 
Davenport, E., corn experiments, 328 

dynamic evolution, 438 

horses, 496, 497 

prepotency, 573 

telegony, 565 
Davis, chromosome counts, 285 

(Enothera, 247, 248 
investigations, 283 
seed sterility in, 283 
Deciduous tree fruits, 393 
Decreased vigor in species hybrids, 230 
Deer, antlers in the male, 214 
Deeringia celosiodes, chimeras in, 381 
Defectives, proportion of, in population, 526 
Defects in domestic animals, 526 

in man, 524 

inheritance of, 524 
"Deficiency," in Drosophila ampelophila, 121, 

194 
Descriptions, need of accurate, 428 
Determination of sex, 536 
Determiners, 71 
Detlefsen, agouti factor, 575 

Bos species hybrids, 520 

Cavia species, 235, 236 

Cavy hybrids, 223, 225, 542, 543 

guinea-pigs, 558 

species hybrids, 236 
Development of the individual, 3 
Dewey, purple-leaved hemp, 270 
Dianthus, hybridized, 438 

vegetative vigor in species hybrids, 230 
Dichotomy, method of, 96 
Differences between variations, 18 

in organisms, 1 

minute, heritable, 451 
Digitalis, 438 

matroclinous Fi in species hybrids, 229 

purpurea X D. ochroleuca, 230 
Dihybrid ratio, 86 

modified, 89 
Dihybridism, 81 

in Drosophila, 87 
Dimorphism fixed, 48, 276 

in plants, 275 
Dioecious plants, 317 
Direction of variations, 20 
Discontinuous variations, 18 

in horticulture, 274 
Disease and animal breeding, 522 

defined, 522 

immunity to, 527 

inheritance of, 522 
Disease-resistance, nature of, 401 

of species, 401 

testing for, 416 
Disease-resistant plants, breeding, 295, 400, 413 

strains, selecting, 417 
Diversity, germinal, in populations, 317 

of ancestry in domestic animals, 460 
Dog, telegony, 565, 570 



Domestic animals, variation in, 454 
characters of, 454 
diversity of ancestry, 460 
germ-plasm of, 459, 460 
Mendelism in, 465 
mutations in, 462 
species hybridization in, 515 
fowl, origin, 461 
plants, varieties of, 302 
Domesticated animals and plants, mutations, 

282 
Domestication and fertility, 551 
Dominance, 69 

and heterosis, 232 
defined, 144 
extent of, 144 

in round and wrinkled peas, 145 
Dominant factors and heterosis, 231 
mutations, 267, 274 
reaction system, 242 
Doncaster, aberrant sex-ratios, 208 
Abraxas, 206 
sex-differentiator, 209 
Dorsey, apple, 393 
Downing, hybridization, 294 
Drosophila, architecture of germ-plasm, 459 
chromosome number in species, 263 
mutations comparatively rare, 267 
reaction system concept, 575 
viability of mutants, 321 
Drosophila ampelophila, abnormal abdomen, 26 
allelomorphic relations in, 145, 146 
bar eye in, 149, 152 
chromosomes in, 57, 58 
different factors producing similar effects 

in, 141 
dihybridism in, 87 
dominant mutations, 267 
eye-color factors, 194, 215 
factor differences causing decreased vigor, 
233 
causing sterility, 234 
factors affecting fertility, 554 
genetic investigations in, 9, 126, 607 
giants and dwarfs in, 130 
inbreeding, effects in, 553 
inheritance of sex in, 66, 67 
lethal factors in, 132, 133 
linkage in, 110, 127, 162 
miniature wings in, 146, 170 
modifying factors for eye color, 192 
multiple allelomorphism in, 156, 161 
mutations in, 29, 154 
non-disjunction in, 198 
pigmentation in, 261 
return mutations in, 154 
segregation in, 64 
sex in, 196 
size differences, 177 
truncate-winged, 168 
white eye color in, 74, 87, 197, 208, 269 
Drosophila melanogaster = D. ampelophila. 
Duchesne, hybrids, 438 
Duchess Shorthorn cows, 556, 580 



656 



INDEX 



Duck, effects of castration in, 550 

Rouen, 548 

secondary sexual characters, 548 
Duplicate factors, 136 
Dwarf beans, 175, 177 

maize, 174 

rootstocks, 383 

sweet peas, 175 



Ear-to-row method, danger of, 334 

in maize, 327 
East, bud selection, 394, 395 
corn breeding, 356 
genotypes in*maize, 326 
germinal variations, 369 
inbreeding in maize, 325 
increased yield from hybrids, 353 
maize, 292, 293, 295 
multiple factor theory of size inheritance, 

185, 194 
Nicotiana hybrids, 228 
regression away from mean, 56 
tobacco, corolla length, 181, 182, 183 
East and Hayes, flint X sweet corn crosses, 83, 84 
heterozygosis, 231 
inbreeding in maize, 325, 353 
inheritance in corn, 101 
Nicotiana crosses, 232 

segregation of starchy and sweet endo- 
sperm, 79 
somatic variation, 272 
tobacco mutation, 177 
vigor in Fi hybrids, 326, 354, 355 
Ecological variations, 18 
Economy of time in breeding maize, 356 
Ectogenetic variations, 20 
Education, service of genetics in, 450 
Effect of castration in animals, 548 
of selection in maize, 333 

in pure lines, 257 
of self-fertilization, 256 
Egg plant, graftage of, 374 
on tomato, effect, 383 
production in fowl, and mass selection, 457, 
498 
and pigmentation, 506 
fecundity, 497 
mean monthly, 505 
sex-linked factor for, 209 
winter, 456, 506, 559 
Elteagnus pungens, 18 

Elderberry, Variegated Black, chimeras in, 381 
Elderton, goodness of fit, 102 
Elliot, Lamarck's laws, 482 
Elm, Fi hybrids in, 365 

Huntingdon, an Fi hybrid, 365 
Emerson, beans, 175 

multiple allelomorphs, 160 
regression away from mean, 56 
somatic variation, 272 
test of hybrid gametes, 159 
Emerson and East, ear-to-row selection, 356 



Endosperm texture, inheritance in maize, 84, 105 
Endothia parasitica, 406 
Englehart, oranges and lemons, 391 
Environment and egg-production, 457 

and plant diseases, 400 

and mutations, 370 

conditions development, 25 

may cause new characters, 26 

modifies development, 21 
Eosin eye color in Drosophila, 156, 192 
Equipment of animal breeder, 450 
Equus caballus X asinus, 228, 516 

hybrids, 468, 520, 557, 567, 570 
sterility of, 557 
Erica, species hybridized, 438 
Erikson, rust resistance, 412 
Erisyphe graminis, 412, 413 
Errors, accidental, 429 

experimental, 429 

probable, 45, 47, 430 

residual, 429 
Eucalyptus globulus, dimorphic leaves, 275 
Eugenics, 11 
Euonymus, 381 
Evans, apple breeding, 295 
Evans, H. M., chromosomes in man, 538 
Evolution and mutations, 276 

and natural selection, 276, 439 

"dynamic," 483 

no longer questioned, 480 

role of factor mutations, 286 
Ewart, domestic animals, 460 

quagga, 566 

telegony, 567, 568, 571 

zebra hybrid, 468 
Exceptional offspring, non-disjunction, 199, 200, 

201, 202, 203 
Experimental breeding, method of, 6 

cytology, method of, 9 

errors, in variety testing, 429 
in reduction of, 431 

morphology, method of, 10 

results, factors affecting, 433 
Extracted dominant or recessive, 73 
Eye color in Drosophila, 194, 215 



Factor, behavior predictable in Pro^ophila, 117 
complex in a breed, 478 

and hybrid vigor, 559 

and prepotency, 575 
conception of heredity, 110 
constancy, belief in, 193, 194 
contamination, assumed by Castle, 189 
cotton leaf, 179 
coupling and repulsion, 126 
differences causing decreased vigor, 233, 554 

causing size differences, 194 
explanation of reversion, 170 
expression, variability in, 134 
for variegation, 273 
interactions, types of, 163 



I 



INDEX 



657 



Factor, interpretation of quantitative inheritance, 
195 
mutations, 263, 264 

at maturation, 209, 270 

cause variegation, 381 

classes, 265 

conception of, 267 

germinal and somatic, 268 

in Boston fern, 315 

in Coleus, 388 

in domestic animals, 462 

in Drosophila, 110. 154, 177, 190, 192, 267. 
462, 554 

in meristematic tissue, 269 

in CEnothera, 285 

nature and causes, 266, 273 

not of fortuitious occurrence, 268 

occurrence of, 371 

produce cultivated varieties, 366 

role in evolution. 286 
potency and prepotency. 575 
relations in quantitative inheritance, 174 
system in stocks, 166 
systems affecting a single character, 195 
variability, by hypothesis of, 192 
Factors, 7. 59, 67, 71 

affecting experimental results, 433 

alterations in, 263, 267 

chemical composition, 267 

coat color in horses, 466 

competitive action of, 150 

complementary, 140 

contamination of, 189 

cumulative. 140 

defined, 129 

duplicate, 136 

first, second, third and fourth groups in 

Drosophila. 117 
for corolla length in tobacco, 183 
for fecundity in fowls, 561 
for high and low protein and oil. 333 
for stature in sweet peas, 175 
four groups in Drosophila, 110 
history in a cross, 70. 73 

in a dihybrid cross. 81 
homologous in species. 224 
in Antirrhinum, 223 
in Cavius. 223 

influencing fertility. 551. 554 
lethal. 131. 546 
linear arrangement of, 115 
linked in same chromosome, 108 
manifold effects of, 133 
modifying. 190. 192, 454 
nature and expression of, 129 
plotted relations in Drosophila. 116 
potency of sex, 217 
producing like somatic effects, 140 
reconabination of, in F;, 186 
relatively stable entities. 194, 267 
sex. 215. 548 

sex-linked in X-chromosome. 196. 546, 548 
system of, in maize, 163 
Fairchild, seed and plant introduction, 299 
42 



Farmer and Digby, chromosomal dimensions, 204 

Primula kewcnsis. 264 
Farrell. experimental error. 430 
Farrcr, grains, 296 

rust resistance, 411 
Fecundity in fowls, 497, 559, 500, 574. 585 
Fern, origin of varieties in Boston, 312 
Fertility and the chromosomes. 555 
and heterozygosis. 559 
capable of improvement, 552 

Darwinian theory of, 552 
factors affecting, 554 
in animals, 551, 563 
in female hybrids, 235, 230 
in species hybrids, 235 
Fertilization, double in maize, 150, 151 

failure of, 350 
Fiber plants, species and varieties, 302 
Fire-blight, disease, 407 
Fischer, modified germ cells. 492 
Flammarion. Coleus, 389 
Flax. 317 

wilt-resistant, 417 
Flower, color in sweet peas, 304 

form and size in sweet peas, 304 
Flowers, composite crossing, 290 

diversity of varieties, 302 

hermaphrodite, self-fertile, 317 
cross-fertile, 317 

hybridized in China, 287 

species hybrids, 297 
Fluctuating modifications, 383 

variations. 18, 32. 428, 439 
Focke, hybrid, 438 

sexuality, 437 

species hybrids, 227, 228, 231 
Foex, phylloxera resistance, 405 
Forms of species hybrids, 227 
Forficula auricularia, 48 
Fortuitous variations, 20 
Four o'clock, see Mirabilis jalapa. 
Fowls, Andalusian, 152, 476, 514, 579 

Bantam, 499 

Barred Plymouth Rock, 504, 574, 575 

Breda, 476 

breeding, for fecundity, 585 

Brown Leghorn. 548 

comb character in, 164 

Cornish Indian Game, 500, 575 

creation of bantam varieties, 500 

dominant white, 579 

effects of alcohol on, 493 
of castration in, 550 

egg production in, 456, 497, 559, 560, 607 

fecundity factors in, 561 

fecundity in, 497, 559 

hybrid, 521 

Mendelian characters in, 477 

Mendelism in, 476 

ovarian transplantation, 487 

phyletic origin, 460 

pigmentation and egg production, 500 

progeny tests, 498 

reversion in. 171 



658 



INDEX 



Fowls, Seabright X Hamburg, 500 

secondary sexual characters, 548 

sex-linked inheritance, 209, 539 

trapnesting, 592 

variation in, 477 

White Leghorn, 478, 579 

White Plymouth Rock, 478. 479, 579 

White Plymouth Rock X White Leghorn, 
478 

Wyandotte, 539 
Fraximus, Fi hybrids in, 365 
Free-martin, 545, 550 

Freeman and Johnson, Puccinia graminis, 412 
Frequency distributions in quantitative inherit- 
ance, 179, 183 

graphs, 39 

table, 38 
Fruit fly, Drosophila ampelophila. 
Fruits, deciduous tree, 393 

disease of pomaceous, 407 

heterosis in, 364 

species crosses in, 297 
and varieties, 302 

top-working poor trees, 391 
Fruwirth, alfalfa, 296 

"Die Zuchtung der landwirtschaftlichen 
Kulturpflanzen," 299 

hybridization, 287 
P^unctional modifications, transmission of, 491 
Fusarium, vasinfectum, trachciphiluni, and 
niveum, 4i3 



Galloway, seed and plant introduction, 299 
Galloway and Dorsett, violets, 391 
Galton, biometrical method, 439 

law of ancestral inheritance, 5, 250, 251 

observational method, 5 

regression, 5, 55, 252 
Galtonian regression, in pure lines, 252 
Gametes, 70, 72, 73 

cross- over, 119 

male and female producing, 540 

non-functional in species hybrids, 234, 243 

number of different kinds in Mendelian 
experiments, 100 
Gametogenesis, 2 
Ganong, Scilla, 25 
Gates, number of chromosomes, 263, 285 

CEnothera, 278 
rubricalyx, 284 

somatic segregation, 274 
Gartner, Nicotiana crosses, 229 

species hybrids, 227, 228, 230, 231, 438 
Garton, grains, 296 
Gaur X domestic cow, 520 
Gayal X domestic cow, 520 
Geddes and Thomson, sex-determination, 544 
Genes, 71 
Genetic analysis, goal of, 608 

constitution, and sex-determination, 218 
of ^2 individuals, 186 
of gametes, 88 



Genetic constitution of pea plants, 70, 71 
of phenotypes, 72, 73, 89 
data, methods of dealing with, 95 
on animals, 608 
recording, 603 
factors, alterations in, 263 

investigation, 609 

knowledge, need of, 609 

ratio, 87, 96. 100, 107, 112. 120 

variability and breeding. 459 
Geneticists' investigations, 610 
Genetics, application of, 11 

content of, 1 

defined, 1 

founded by Mendel, 439 

in agriculture, 12 

preroqviisites for, 10 

problems of, 4 

relation to plant breeding, 440 

service of, 448, 450, 609 
Genotype, 72 
Genotypes, in fowls, 587 

in maize. 326 

in self-fertilized population, 320 

in wheat, 336 

not all pure lines, 256 

number of kinds in F^, 100 

progression method of computing, 97 
Genotypic selection at Svalof, 425 

fundamental in breeding, 262 

in animal breeding, 584 

registration rules favor, 496 
Geranium, natural chimera in, 379 

variegation, 381 
Germ cells, 57, 70, 73 

combination of chroniosonios in, 65 

formation, 72 

production of, 60 
Germinal, diversity in populations, 317, 462 

elements of domestic animals, 460 

recombinations and mutations, 453 
Germ-plasm and soma, 2, 485, 490 

architecture of, 459 

continuity of, 485, 488 

isolation of, 488 
Gernert, teosinte hybrids, 410 
Geum, vegetative vigor in species hybrids, 230 
Gibbs, seed and plant introduction, 299 
Gideon, apple breeding, 295 
Gigantism due to factors, 194 

due to tetraploidy, 264 
Gilmore, timothy, 294 
Gladiolus, 438 

cardinalis and G. tristis, 271 

colvillei, "The Bride," 270 

chimeras, 381 

species and varieties, 302 
Glaser, chromatin. 264 

Glucosids, transmission from stock to scion, 384 
Glycogen, stereoisomeric forms, 266 
Goats, sex-determination in, 536 
Godron, wheat. 438 
Goldschmidt, effect of environment, 22 

free-martin, 545 



INDEX 



659 



Goldsehmidt, gypsy moth, 217, 575 

Lymantria, 216, 546, 558 

population, 260 

potency of a sex-factor, 217, 218 
Goodale, ducks and fowls, 550 
Goodness of fit in Mendelian ratios, 100 
Goodspeed and Clausen, Nicotiana crosses, 238, 
240 

reaction systems, 242 
Goss, pea crosses, 294 
Grading in animal breeding, 508, 583 

precautions in, 512 
Graft components, 374, 378, 383 

hybrids, 374 
behavior, 382 
historical cases, 378 
tomato-nightshade, 374 

infection, 381 

-symbionts, modification of, 383 
Grafting and graft-hybrids, 374, 376 

of insects, 382 

of ovaries in fowls, 487 
in guinea-pigs, 485 
Grains, composite crossing of, 296 
Grape chimeras, 381 

collections, 299 

Concord, 290 

earliest American hybridizers, 290 

heterosis in, 364 

stocks, hybrid, 408 
Grapes, immunity to phylloxera, 404 

resistance to phylloxera, 402 

resistant stocks, 404, 408 

wine, effect of grafting, 383 
Grasses, at Svalof, 293 

remnant system, 335 
Grassi, phylloxera, 405 
Gregory, linkage in Chinese primrose, 125 
Grew, sexuality, 437 
Grouse locust, Paratettix. 
Guignard, stock and scion, 384 
Guinea-pigs, agouti pattern, 225 

effects of alcohol on, 493 
of castration in, 550 

hybrid, 223, 235, 542 

inheritance of coat characters, 85 

Mendelian inheritance, 224 

ovarian transplantation, 485 

polydactylous, 553 
Guthrie, fowls, 487 
Guyer, chromosomes in man, 538 

defects in man, 524 

sex in fowls, 539 
Gynandromorphism, not due to mutation, 268 
Gypsy moth, intersexes, 217, 575 

H 

Habit in sweet peas, 308 

Haemophilia, 524 

Hadley, white leghorn, 478 

white Plymouth rock, 479 
Hagedoorn, pure lines, 258 
Hall, experimental error, 430 



Hall and Russel, experimental error, 430 
Hallet, plant breeding, 437 

selection, 366 

wheat, 289 
Hansen, N. E., alfalfa, 296 

apples, 297 

pear blight, 407 

seed and plant introduction, 299 

resistant fruits, 408, 411 
Hansen, Barber, Wolf and Jordan, mutations in 

bacteria, 27 
Harris, egg production, 507 

milk yield, 55 

ratios, testing goodness of fit, 102 

seed production, 442 

soil heterogeneity, 430, 431 
Harris, O., cattle, 446 
Hartley, bud sport, 273 

corn, 293 

plant-to-row-strain, 433 

selection, 335 

somatic variations, 272 
Hayes, tobacco, 367, 368 
Hayes and East, factors, competitive action of, 

150 
Hayes and Jones, cucumber crosses, 362 
Hays, corn, 358 

grain breeding, centgener method, 293 

high protein, 359 

hybridization of plants, 294 

pedigree cultures, 419 

pure lines, 339 

selection, 366 
Hays and Boss, wheat inflorescence, 347 
Headden, yellow berry in wheat, 418 
Heape, sterility, 552 
Hedera helix, dimorphic leaves, 275 
Hedrick, apple breeding, 295 
Helianthus lenticularis cotonatus, 270 
Hemoglobin, stereoisomeric forms, 266 
Hemp, 318 

heterosis in, 364 

purple leaved, 270 
Henri, pear-quince hybrid, 378 
Henry, tree hybridization, 365 
Herbert, Erica, 438 

species crosses, 230, 438 
Herd books, a source of Mendelian data, 465 

Shorthorn, 597 
Hereditary materialof domestic animals, 444 
Heredity, 1, 57, 439' 

defined, 3 

problems of, 4 

methods of attacking, 4 

studies require funds, 609 
require time, 608 

study of revolutionized, 439 
Hcribert-Nilsson, Oenothera, 247 
Heritability of variations, 15 
Hermaphrodites in Lychnis dioica, 213 

mutants, 215 
Heterogeneity in species, 250 
Heterosis, and partial sterility, 365 

explained, 232 



660 



INDEX 



Heterosis in species hybrids, 233 

in trees, 365 
manifestations of, 353 

utilization in corn, cucumber, tomato and 
tobacco, 362 
in vegetatively propagated plants, 364 
Heterosynapsis, 201, 204 
Heterozygosis, and fertility, 559 

and hybrid vigor, 231 
Heterozygotes, contamination of factors in, 189 

number of, in Ft, 100 , 

Heterozygous genotypes, 72 

number of kinds of, in Fi, 100 
Heuer, Solanum chimera, 378 

Solanum grafts, 374 
Hibiscus, dimorphic leaves, 275 
Hinny, 517 

sterility of, 556 
Hippeastrum, hybridized, 438 
History of animal breeding, 443 

of plant breeding, 287 
Hoffman, hybridization, 439 

Hoge, Miss, reduplication of legs in Drosophila, 
272 

supernumerary legs in Drosophila, 136 
Homeosis due to mutilation, 269 
Homeotic variations, 16 
Homosynapsis, 201, 204 
Homozygotes, number of, in Fi, 100 
Homozygous genotypes, 72 

number of kinds, in Fi, 100 

of tobacco, 182 
Honey bee, 26 

sex-determination, 210 
Hops, heterosis in, 364 
Hopkins, corn breeding experiments, 327 

maize, 293 
Hopkins and Smith, corn experiments, 328 
Hormones and dimorphism, 276 
Horse, American Standard bred, 484, 495, 574 

chromosome number in, 519, 537 

fertility in, 552 

history of modern, 460 

hybrids, 516, 528 

-quagga hybrids, 565 

secondary sexual characters in, 548 

sex-determination in, 536 

spermatozoa in, 541 

telegony, 565 

-zebra hybrids, 468, 520, 557, 567, 570 
Horses, Arabian, 444, 460, 566, 596 

Barbs, 444, 460 

coat color in, 465 

Clydesdale, 460, 461 

creating a gray breed, 470 

Danish, 444 

diversity of ancestry, 460 

Mendelism in, 465 

Percheron, 444, 460 

racing, 496 

Shetland, 460, 489 

Shire, 460 

Standard bred, 465, 467 * 

sterility in, 552 



Horses, Suffolk Punch, 460, 466 
Thoroughbreds, 461 
types of, 460 
white dominant in, 579 
Horticultural practice, bud selection in, 391 
Hoshino, Unkage in peas, 125 
Hover, prepotency, 574 
Hovey, hybridization, 294 
plant breeding, 437 
strawberry, 289 
Hume, peach, 297 
Humming birds cross tobacco, 319 
Huntington's chorea, 525 
Hunt, timothy, 294 
Hurst, horses, 465 
sweet pea, 310 
Hutcheson, selection, 339 
Hyacinth, improvement by Dutch, 288 
introduction, 287 

species and varieties, 302 
Hyalodaphnia, 23 
Hybrid, first plant, 438 

seed corn, producing, 361 
Hybrids, aphis resistance in, 410 

cavy, 223 

forms of species, 227 

graft, 374, 382 

increased production in Fi, 353 

intergeneric, 296 

intermediate expression in, 147 

interspecific, 296 

mosaic expression in, 152 

multifactor, 94 

Nicotiana, 228, 229 

CEnothera, 244 

sterility of, 556 

in species, 234, 236, 238 

superior quaUties in Fi, 359 

utilization of, in breeding, 353 

variable character, expression in, 149 

vigor of species, 230 
Hybridization, 11, 291, 294, 342 

and animal breeding, 508 

at Svalof, 425, 427 

begun by Knight, 288 

castration, 343 

choice of parents, 343 

conditions favorable for, 350 

difficulties in, 350 

extensive use, 311 

for disease resistance, 408 

future promise of, 294, 312, 316, 352 

general method, 343 

Goss and Mendel, 294 

harmful effects of, 571 

in apples, 295 

in birds, 521, 543 

in cattle, 473, 475, 520, 531, 533, 542 

in Coleus, 386, 389 

in cotton, 295 

in creating varieties, 302, 311 

in grain breeding, 294 

in guinea-pigs, 223, 235, 542 

in horses, 516, 528 



INDEX 



661 



Hybridization in maize, 295 

in sweet peas, 309 

in the rose, 310 

of alfalfa, 348 

of maize, 344 

of wheat, 345, 347 

pollination, 343 

practised by Thaer, 288 
in China, 287 
in Holland. 287 
in Italy, 287 

protection of flowers, 344 

species, 219, 249, 342, 351, 515 

systematic procedure, 343 

variety, 248 
Hyde, fertility, 553 

Hydrogen ion concentration in plants, 413 
Hyperchimera, 375 

Hypothesis of factor contamination in heterozy- 
gotes, 190 

of factor variability, 192, 193 

of reaction systems, 242, 248, 575 



I 



Ideal of breeder, 342 

Ideas, erroneous in breeding, 449 

Illinois corn-breeding experiments, 327 

role of selection in, 329, 333 
Immunity, acquired, 528 

breeding for, 529 

indirect, 528 

to disease, 404, 527. 
Inbreeding, as a system, 581 

coefficient of, 598 

curves of, 600 

Darwin's idea of, 553 

in maize, 325 

not harmful in itself, 553 
Increased vigor in species hybrids, 230 
Index numbers, selection, 334, 502 
Individual plant selection, 340 
Infection of male, 571 
Infectious chlorosis, 381 

IngersoU, increased yield from hybrids, 354 
Inheritance, blending, recognized by CEistle, 188 

critical problem of, 4 

intensity of, 459 

of acquired characters, 6, 480 

of aleurone color in maize, 83, 105 

of black and curved in Drosophila, 113 

of coat color in horses, 467 
in mice, 94 

of coat characters in giunea-pigs, 86 

of corolla length in tobacco, 181 

of defects. 524 

of disease, 522 

of effects of use and disuse, 491 

of endosperm texture in maize, 84 

of fecundity in fowls, 560 

of flower characters in snap dragons, 90 

of larval pattern in silk worm, 209 

of pigmentation in hooded rats, 186 

of predisposition to disease, 523 



Inheritance of sex, 65, 196, 204, 205, 211. 212 

of size differences, 185 

of unusual sex-ratios, 546 

of vestigial wings, 87 

of white-eye color, 74 

quantitative, 174, 185, 194 

three types recognized by Castle. 189 
Insects, grafted. 3S2 

injurious. 400 

sex-determination in, 210 
Interactions, types of factor, 167 
Interference, 122 
Intermediate, condition in Fi species hybrids. 227 

expression in the hybrid. 147 
Internal secretions and dimorphism. 276 
Intergeneric hybrids. 296 
Intersexes, in gypsy moth, 217 

in Lymantria, 216 
Interspecific hybrids, 296, 297 
Introduction, seed and plant, 299 
Investigation, genetic, 4, 591, 609 

methods of conducting, 591 

need of biochemical, 413, 485 

of a natural chimera, 378 
Isolation of pure lines from populations, 256 



Janssens, crossing-over mechanism, 115 
Jeffrey, sterility of hybrids, 234 
Jennings, development, 10 

regression, 56 
Jesenko. wheat-rye hybrids. 236. 237, 238 
Johannsen, bean mutations, 367 

gene, 71 

genotype conception, 329 

mathematics in biology, 56 

Princess bean, 250 

pure lines, 7, 252, 253, 254, 255, 256 

regression, 56 

selection. 250, 366 

summary of Mendel's results, 79, 80 

variation, 439 
Johnson, tobacco, 418 
Jones, A. X., grains, 296 
Jones, D. F., heterozygosis, 559 

vigor of hybrids, 231, 326, 364 
Jones and Hayes, corn, crossing varieties, 360 

tomatoes, 363 
Journal of Heredity, 300 
Judging the individual, 591 
Juglans caUfornica var. quercina, 265 

californica X nigra, 297, 364 

californica X regia, 364 

nigra X regia, 364 



Keeble and Pellew, heterosis, 232 

vigor in F\ hybrids, 326 
Keeping varieties pure, 341 
KeUerman, Miss, preserving pollen, 350 
Kent, correlation in poultry, 507 
Khang-hi, imperial rice, 287 _ 



662 



INDEX 



Kildee and McCandlish, dairy cattle, 455 

grading, 509, 510, 511 
King, Miss, sex-ratio in rats, 542, 543, 547 
King, biometrical accuracy, 38 

law of probability, 34 
Klebs, effect of light on plants, 21, 22 
Klotzsch, tree hybridization, 365 
Knight, apple breeding, 295 

fertility, 438 

hybridization, 288, 294 

plant breeding, 437 

wheat rust, 411 
Kolreuter, Nicotiana, 228, 438 

species hybrids. 227, 230, 438 
Kuhlman, Jersey-Angus cross, 474 
Kyle, comparing yields, 358 



Labeling, system of, 423 
Laburnum-Cytisus graft-hybrid, 378 

vulgare, 378 
La Gasca, wheat varieties, 289 
Lamarck, acquired characters, 482 

evening primrose, 277 

theory of evolution, 14 

use and disuse, 491 
Lane, dairy records, 592 
Lang, population, 260 
Lankester, Lamarck's laws, 482 
Larix, Fi hybrids in, 365 
Lathyrus odoratus, 303 
Lavatera hybridized, 438 

vegetative vigor in species hybrids, 230 
Law of ancestral inheritance, 5, 250, 251 

of deviations from the average, 34 

of filial regression, 5, 55, 252 

of statistical regularity, 32, 33 
Leake, cotton leaf factor, 178, 179, 181 
Learning, maize, 292 
Lecoq, species hybrids, 230 
Le Couteur, progeny test, 293 

plant breeding, 437 

selection, 366 

wheat varieties, 289 
Legumes, 317 

species and varieties, 302 
Leidner, rye inbreeding, 319 
Lemon bud selection, 391 

chimera, 271 

Eureka, 392 

performance records, 392 

shade-tree type, 392 
Leptinotarsa decemlineata, 30, 248 

selection in, 258, 260 
Lethal factors, 131, 546 
Lillie, barrenness in free-martin, 550 

free-martin, 545, 546 
Lily, species and varieties, 302 
Lime, common, an Fi hybrid, 365 
Line breeding, 582 

selection, 291, 293, 340 
Linear arrangement of factors in the chromo- 
somes, 115 



Linkage, 104 

affecting quantitative inheritance, 186 

and chromosome theory of heredity, 104 

and composition of populations, 321 

and heterosis, 232 

and non-disjunction, 202, 204 

and prepotency, 576 

between purple aleurone and waxy endo- 
sperm in maize, 105 

chromosome interpretation, 108 

in Drosophila, 110 

in silkworm, 209 

in sweet peas, Chinese primrose, garden 
peas, snapdragon, oats, rats, silkworm, 
125 

method of determining, 127 

partial, 106 

relations in Mendelism, 104 

values predicted, 117 
variations in, 120 
Linnaeus (von Linn6), hybrids, 437, 438 
Linum, 415 

hybridized, 438 
Lion, mane, 214 
Little and Phillips, mice, four Mendelizing factors 

in, 94 
Livestock, diverse ancestry, 493 

pure bred, 446, 450 

value of, 445 
Lloyd-Jones, fertile mule, 518, 519 
Lloyd-Jones and Evvard, color in cattle, 471, 472, 

473, 474 
Localization of cotton varieties, 440 

of plant breeding, 440 

of selected maize, 361 

of truck crops, 440 
von Lochow, rye, 293 
Lobelia, 438 

cardinalis X L. syphylitica, 230 

vegetative vigor in species hybrids, 230 
Loeb, Bryophyllum calycinum, 276 
Lotsy, Antirrhinum, 220, 222, 227, 248 

Nicotiana crosses, 229 
Love and Leighty, variation, fluctuating and 
stable, 55 

variation in oats, 43 

yield of plants, 38 
Lutz, Miss, CEnothera chromosome counts, 285 
Lutz, dominance incomplete, 146 
Lychnis, dioica typica X angustifolia, 214 

hermaphrodite mutants, 215 

multiple allelomorphs in, 160 

sex-inheritance, 196, 213 

sex-linked characters, 214 

vegetative vigor in species hybrids, 230 
Lycium, hybridized, 438 
Lymantria, dispar X japonica, 216 

intersexes, 216, 546, 558 
Lyon, experimental error, 430 



M 



MacDougal, experiments on plants, 27, 28 
Mackie, rust resistant grain, 412, 413 



INDEX 



663 



Macoun, apple breeding, 295 
Maize, 317 

aphis resistant hybrids, 410 

bud sports, 273 

centralized seed production, 361 

chromosome number, 57 

collecting stock seed, 341 

competitive action of factors for flinty and 
floury endosperm, 150 

complementary factors in, 140 

crossbreeding, 354 

crossed with tcosinte, 296, 410 

crossing inbred strains, 354 
varieties, 356, 358, 360 

dominant mutations, 274 

double fertilization in, 151 

ear-to-row method of breeding, 327, 433 

economy of comparing yields, 356 

factors for protein and oil, 333 

grain, physiological characters, 333 

high and low ears, 329 
and low oil, 328 
and low protein, 328 

hybridization of, 344 

Illinois experiments, 327, 330, 331, 333 

immediate effect of crossing, 360 

inbreeding in, 325, 354, 356 

inheritance of starchy and sweet endosperm, 
79 
of purple and white aleurone, 81 

mass selection in, 292, 336 

method of comparing yields, 356 
of producing hybrid seed, 361 

multiple allelomorphs in, 159 

mutations in, 367 

naturally cross-fertilized, 257 

pure line concept and, 293 

purple sweet X white starchy, 84 

remnant system, 335 

role of selection in breeding, 333 

selection in breeding, 325 

superior qualities in F\, 359 

system of aleurone color factors in, 163 

tall and dwarf, 174 

use of Fi seed, 356 

vigor in Fi hybrids, 231, 233, 253 

water requirement, 402 

yield of inbred strains, 354 
Malva hybridized, 438 

mauritiana X M. sylvestris, 230 
Malvacese, variegation in, 381 
Mammals, sex-determination in, 536 
Man, color-blindness, 197 

defects in, 524 

disease immunity in, 528 

sex-determination in, 538 

sex-inheritance, 196 

sex-ratios in, 540 
Manifold effects of factors, 133 
Marking indi\'idual animals, 602 
Marryat, Miss, Mirabilis jalapa, 148 
Marshall, F. H. A., castration effects, 549 

"Physiology of Reproduction," 551 

sterility, 552 



Marshall, F. R., Redfield theory. 483 
Mass selection, 260, 291 
in cotton, 292 
in fowls, 457 
in maize, 292, 336 
in small grains, 292 
usefulness, 340 
Material, plant breeding, 299 
Maternal impressions, 572 
Mathematical adequacy of Mendelism, 77 
Mathiola, hybridized, 438 

Miss Saunder's factor system in, 166 
Matroclinous species hybrids, in Digitalis, 229 

in CEnothera, 229 
Mauz, species hybrids, 230 
Mayer, chimeras, 378 
McCann, studbook records, 466 
McCombie, breeder, 570 
Mean, calculation and significance, 40 

of an F2 population, 184 
Mechanism, segregation, 61, 274 
Medicago falcata, 296 
lupulina, 296 

media (falcata X sativa), 296 
sativa, 296 
Medlar-whitethorn chimera, 378, 379 
Meek, chromosomal dimensions, 264 
Melandrium, hybridized, 438 
Mendel, experiments with peas, 69, 77, 78, 144 
investigations, 1 
peas, 174 

principle of segregation, 104, 110 
re-discovery, 438-439 
segregation discovery, 68 
MendeUan, analysis of CEnothera phenomena, 
247 
of wheat;rye hybrids, 238 
characters, 71 
conceptions, modern, 242 
experiment, goodness of fit in, 101, 102 
explanation modified, 248 

of Antirrhinum cross, 222 
factor, 71 

for variegation, 273 
factors, affecting fertility, 554 

nature and expression of, 129 
heredity, 71 
inheritance, 57, 59, 188, 224, 333 

independent, 68 
interpretation of heredity, 67, 77 

of prepotency, 574 
"laws" modified and extended, 439 
principles, application of, 342 
establishment of, 85 
operation of, 70, 515 
principles and quantitative inheritance, 174 
problems, complexity, 100 
ratios, forces tending to disturb, 102 
methods of testing, 100 
peculiar, 132, 137, 139, 140, 141, 142 
terminology, 71 
theory of heredity, 7 

of sex, 196, 205, 544, 546 
Mendelism, cardinal feature of, 68 



664 



INDEX 



Mendelism in domestic animals, 465 

mathematical adequacy of, 77 

operation of, 68 

physical basis of, 57 

principle discovered by Naudin, 439 

rapid development of, 71 
Mendelizing characters, due to factor mutations, 
286 

in domestic fowls, 477 
Mercer and Hall, experimental error, 430 
Meristic variations, 16 
Mespilus-Crataegus graft-hybrids, 378 

germanica, 378 
Metabolic theories of sex, 544 
Method of comparing yields, 356 

of cytology in genetic research, 8 

of experimental breeding, 6 
morphology, 10 

of hybridization, general, 343 

of observation in genetic research, 4 

of producing hybrid seed corn, 361 
Methods of breeding animals, 577 

of breeding investigations, 591 

of creating new varieties of plants, 302 

of dealing with genetic data, 95 

of pedigree culture, 419 

of planting, 420 

of protection, 420 

of recording and preserving data, 422 

of seedage, 419 

of testing Mendelian ratios, 100 
Metz, Drosophila, 263 
Meyer, seed and plant introduction, 299 
Meyer and Schmidt, stock and scion, 384 
Mice, affected by temperature changes, 491 

disease in, 527, 529 

effects of alcohol on, 493 

four-fold factor segregation in, 94 

multiple allelomorphs in, 155, 160, 225 
Miles, factor variability, 134 
Milk, efficiency, 503 

production records, 445, 455, 591 
Milo, resistant to smut, 416 
Milward, certified seed, 397 
Miniature wings in Drosophila, 146, 170 
Minot, sex-ratio, 542, 543 

Mirabilis jalapa, intermediate expression in 
heterozygotes, 147 

vigor in species hybrids, 230 
Mitosis, 59 

and somatic segregation, 274 

mechanism of, 274 
Mixed populations, isolation of pure lines, 256 
Mode defined, 34 
Modifiability and breeding value, 456 

and correlation, 459 

and selection, 578 

and variation, 454 

in fecundity, 498 
Modification of graft-symbionts, 383 
Modifications, 15 

functional, inherited? 481, 489, 491 

not transmissible, 385 

of phytomeres, 385 



Modifying factors, assumed in hooded rats, 190 

discovered in Drosophila, 192 
Moenkhaus, Drosophila, fertility, 553 
Mohler, zebu immunity, 528 
Molecular configuration and specificity, 266 
Mongrel dairy cows, 455 

ponies, 567 
Monoecious plants, 317 
Monohybrid, 68 

ratio, 71 
van Mons, plant breeding, 437 

systematic breeding, 288 
Montgomery, E. G., experimental error, 430, 431 

wheat, 432, 433, 434 
Montgomery, T. H., chromosomes in man, 538 
Morgan, abnormal abdomen in Drosophila, 26, 
134 

crossing-over, unit of, 117 

dominant mutant factor, 149 

effects of ether on Drosophila, 74 

eye color in Drosophila, 194 

factor group, determination, 115 
relations, 575 

factors, lethal, 132 

manifold effects of, 133, 134 
similar effect of, 141 

gynondromorphism, 269 

heredity, factor conception of, 110 

hickory phylloxeran, 211 

lethal factor, 547 

linkage, 110, 116 

Mendelism, 68 

multiple allelomorphism, 155, 160 

presence and absence hypothesis, 155 

sex-ratio, 540 

sterility, 554 

three-point experiment, 118 

white eyes in Drosophila, 74 

WZ type of sex-inheritance, 205 

XY type of sex-inheritance, 196 
Morgan and Bridges, crossing-over, 113 

data, three-point experiment, 118 

Drosophila mutation, 555 

three-point experiment, 123 
Morphological variations, 16, 17 
Morrow and Gardener, increased yield from 

hybrids, 354 
Morse, sweet pea, 208 
Morton, quagga hybrid, 565 
Mosaic expression of the hybrid character, 152 
Mosquito, yellow fever, 528 
Moths, grafted, 382 

intersexes in, 216, 217 

sex-inheritance in, 205, 209 
Mucuna utilis, valuable mutations in, 367 
Mule, breeding industry, 516 

chromosomes in, 559 

Equus caballus X E. asinus, 228 

fertility of, 518 

reduction divisions in, 558 

sterility of, 556 
Mullein thrips, see Anthotrips verbasci. 
MuUer, crossing-over, double, 124 
factors for, 120 



INDEX 



665 



MuUer, Drosophila, multiple recessive females, 
121 
fertility, 554 
interference, 122 

linkage in 4th group of Drosophila, 117 
Muller and Bridges, crossing-over, time of, 121 
Multi-factor hybrids, 94 
Multimodal curves, 48 
analysis of causes, 49 
conditions of, 48 
in F2, 179, 181 
Multiple allelomorph hypothesis, 267 
allelomorphism, 155 
allelomorphs in Drosophila, 155 
in fowls, 575 
in mice, 155 
in rabbits, mice, Aquilegia, Lychnis beans, 

160 
in silkworm, 158, 210 
factors, 136 

theory of, in quantitative inheritance, 185, 
186 
Mumford, hinny, 517 

telegony, 570 
Munson, bud selection, 298, 391 
grapes, 290 
hybridization, 294 
Mutants dominant, 267, 274 
generally recessive, 267 
hermaphrodite, 215 
in hooded rats, 190 
occurrence of, 372 
propagation of 372 
reversible sex, 213 
so-called in (Enothera, 248 
white-eyed, discovery of, 74 
"Mutants," classification in Oenothera, 285 

results of crossing Oenothera, 247 
"Mutating" species are hybrids, 285 
Mutation and environment, 370 
bud, in cherry, 383 
in evolution, 282 
restriction of term, 286 
theory, 7, 8, 276 

vegetative vs. somatic segregation, 272 
Mutations, 15, 263 

affecting size, 177, 304 

and evolution, 276 

bud, 311, 315, 385 

classes, 263 

dominant, 267, 274 

factor, 110, 263, 264, 267, 268, 285, 304 

in barley, wheat, potato, tomato, 369 

in beans, 367 

in Boston fern, 315 

in breeding sugar beets, 340 

in cotton, hemp, rye, sunflower, 370 

in crop plants, 367 

in domestic animals, 462, 494 

in Drosophila comparatively rare, 267 

in homologous factors, 273 

in maize, 369 

in modifying factors, 190 

in oats, 367, 371 



Mutations in one of a pair of factors, 273, 274 

in plant breeding, 366 

in sweet peas, 304 

in tobacco, 367 

progressive and degressive, 315 

propagation of, 372 

return, 154, 213 

search for, 370 

vegetative, 270 
''Mutations," de Vries' classification, 282 

in the Evening Primroses, 276 

recombinations, 282 
Mutilation, susceptibiUty of some flowers, 350 
Mutilations, inheritance of, 483 
Myers, strain tests, 433 



N 



Nabours, mosaic hybrids, 152 
Paratettix, 158, 159 
zebu, 531 
Nageli, hybridization, 438, 439 
Narrow breeding, 334 
Nathusius, Bos species-hybrids, 520 

mutations in sheep, 462 
Natural chimera, investigation, 378, 381 
hybrids in autogamous species, 256 
selection, 5, 276, 286, 439 
Nature of disease-resistance, 401 

of variations, 16, 317 
Naudin, species hybrids, 227 
wheat-spelt hybrid, 438 
Neo-Lamarckians and acquired characters, 483 
Nephrolepis exaltata Bostoniensis, 312, 314 
Ameropohli, 314 
Craigi, 314 
Dwarf Boston, 313 
elegantissima, 313 
Goodi (gracilUma), 314 
Harrisi, 315 
magnifica, 313, 314 
Millsii, 313 
muscosa, 313 
Piersoni, 312 
Randolphi, 315 
Roosvelti, 313 
Scotti, 313 
superbissima, 313 
Teddy Jr., 313 
verona, 313 
viridissima, 313 
Whitmani, 314 
Wm. K. Harris, 315 
germinal mutations in, 316 
Newman, creating populations, 352 

pure varieties, 341 
" New creations," 441 
New varieties, earliest systematic production, 287 

methods of creating, 302 
Niata cattle, 276 

Nice, effects of alcohol on mice, 493 
Nicotiana alata grandiflora, 319 

haploid chromosome number, 242 
longiflora, 182 



666 



INDEX 



Nicotiana, partial sterility in species hybrids, 
238 

rustica X N. paniculata, 438 

species hybrids, 227, 228, 229, 232 

sylvestris, illustrated, 239 

tabacum, mutations in, 367 

tabacum X sylvestris, 218, 238, 244, 575 

vigor in species hybrids, 230 
Nicotine, transmission from stock to scion, 384 
Nightshade, graftage of, 374 

species hybrids, 439 

-tomato graft-hybrids, 374 
Nilsson, pedigree culture, 419 

selection, 366 

Svalof system, 425 

Vilmorin method, 293 
Nilsson-Ehle, creating populations, 350 

duplicate factors, 137 

oats, 368 
Nomenclature, confusion in, 428 
Non-disjunction in Drosophila, 198, 204 

primary, 199 

secondary, 200 
Non-functional gametes and zygotes, 248 
Non-Mendelian inheritance, 248 

no clear evidence for, 248 
Normal curve and its significance, 34 
Norton, Jesse B., Puccinia asparagi, 415 
Norton, J. B. S., color chimera, 272 

tomato wilt, 415 
Nucleus, 59, 61, 62 
Nucleolus, 60, 61 
Nursery stock, pedigreed, 394 

recorded, 394 
Nuts, breeding disease-resistant, 410 

heterosis in, 364 



O 



Oak, Fi hybrids in, 365 
Oats, 317 

autogamous, 257 

Banner, 340 

Imported Scotch, 340 

Irish Victor, 340 

linkage in, 125 

little natural crossing, 318 

mutations in, 367, 371 

pure lines, 293, 340, 371 

selection of pure lines, 371 
Obrecht, mule, 516 

Observation, method of, in genetic research, 4 
(Enothera, albida, 281, 282, 285 

biennis X O. franciscana = neo-lamarckiana, 
248, 283 

brevistylis, 279, 282, 284 

crossbreeding experiments, 283 

cytological studies in, 285 

factor mutations in, 285 

genetic factors, 283, 284 

gigas, 280, 282, 285 

investigations classified, 283 

IsevifoUa, 278, 282, 285 



ffinothera, lamarckiana, chromosome number, 
285 

described, 276 
family pedigree, 279 
hybrid, origin of, 282 
lamarckiana X O. rubrinervis, 245 
lata, 279, 282, 285 
matroclinous species hybrids, 229 
Mendelian inheritance in, 283, 285 
"mutations," 276 
nanella, 279, 282, 285 
nanella X O. rubrinervis, 245 
oblonga, 281, 282, 285 
pangens, 247 

progressive, degressive, regressive and in- 
constant species, 281 
rubricalyx, 284 

dominant mutant, 267, 270 
rubrinervis, 280, 282, 285 
rubrinervis X O. nanella, 247 
scintillans, 281, 282, 285 
seed steriUty, 283 
species hybridization, 244 
Okra, dimorphic leaves, 275 
Olive chimeras, 381 
Oliver, hybridizing alfalfa, 348, 349 
Olmstead, experimental error, 430 
Orange, Australian navel, 271 
bud selection, 391 
chimeras, 375 
corrugated, 392 
performance records, 391 
ribbed, 392 
Thomson navel, 392 
Washington navel, 391, 392 
yellow navel, 392 
Organic evolution and chromosome development, 
264 
role of factor mutations, 286 
Organization of plant breeding, 299 
Organizations of plant breeders, 300 
Origin of Boston fern varieties, 312 
of domestic animals, 460 
varieties of plants, 302 
of sweet pea varieties, 303 
of varieties of sorghum, 370 
Ornamental trees, rapid growing, 365 
Orthogenetic variations, 20 
Orton, cotton, 417 

Fusarium niveum, 415 
potato, 416 
watermelon, 414 
Outbreeding, 583 



Paelinck, stock and scion, 383 

Pangens, 246 

Parallel induction, 492 

de Parana, zebra hybrids, 570 

Paratettix, mosaic expression in, 152 

multiple allelomorphs in, 157, 160 
Parents, choice of in hybridization, 343 

culture of in hybridization, 343 



INDEX 



667 



Parental forms, recovery of in Fz, 176, 180, 184, 

186, 242 
Partial linkage, 106 

sterility in hybrids, 235, 236, 238 
Pathogene, fire-blight, 407 
Pathogenic fungi and bacteria, 401 
Patten, apple breeding, 295 

resistant fruits, 408 
Pattern, face, in Hereford cattle, 454, 474, 534 
Pea, 317, 440 

Marrowfat, 290 

sweet, 303 
Peach-almond graft-hybrid, 378 

and almond, grafting, 383 

breeding, 297 
Pear breeding begun, 288, 295 

chimeras, 381 

Chinese resistant to fire-blight, 408 

discontinuous variation in Le Conte, 274 

fire-blight resistance, 407 

-quince graft-hybrid, 378 

resistant hybrids, 407, 411 
Pearl, alcoholized fowls, 493 

biometry, 6 

coeflBcient of inbreeding, 598 
of relationship, 601 

curve of inbreeding, 600 

directions for poultry breeding, 590 

egg production, 209, 456, 464, 578, 588, 589, 
607 
index, 593 

fecundity in fowls, 559, 560, 561, 562, 563, 
574 

fowls, 476, 497, 499, 500 

genotypic selection, 585 

labeling system, 423, 424 

milk record, 455 

natural phenomena, causes of, 268 

potency, 575 

prepotency, 576 

problem of inheritance, 4 

Redfield's theory, 483 

selection indices, 502, ,503 

sex-ratio, 544 

statistical knowledge, 55 
Pearl and Bartlett, characters of the corn grain, 

333 
Pearl and Parshley, sex-ratio, 543, 544 
Pearl and Surface, bean and oat breeding, 318 

fowl, 209 

monthly egg production, 505, 506 

selection index numbers, 334, 504 

trap nest, 592 

winter egg production, 457 
Pearson, biometry, 5 

prepotency, 576 

saturation, 571 

tables for statisticians, 102 

variability index, 53 
Peas, at Svalof, 293 

crossing in, 318 

Mendel's investigations with, 77 

pure lines in, 340 

tall and dwarf crossed, 68, 174 



Pedigree culture, 6, 419 
in cotton, 295 
methods, 419 
system of, 293 
method, in Coleus, 390 
breeding, 580 
"Pedigreed" nursery stock, 394 

trjees, 394 
Pedigrees, ancient, 444 
of fowls, 588, 589 
livestock, 596 
Pelargonium zonale, chimera, 379 
Pentstemon, vegetative vigor in species hybrids, 

230 
Peony collections, 299 
Performance records of fruits, 391 
Periclinal chimera, 375, 376, 380 
Petunia nyctaginiflora X P. phcenicca, 230 
Phaseolus vulgaris nana, 250 
Pheasant hybrids, 521, 558 
Phenotype, 72 

Phenotypes in dihybrid Fi, 82 
Phenotypic ratio, in a cross involving a sex- 
linked lethal factor, 133 
in dihybrid, 82 

in Fi with n pairs of factors, 98 
in F2 of purple starchy X white waxy 

maize, 106 
in F2 of purple wavy X white starchy 

maize, 108 
in Fz of yellow red $ X gray white c? in 

Drosophila, 113 
in Fi of yellow white 9 X gray red cf in 

Drosophila, 112 
in Fi of dihybrid, 85 
in monohybrid, 71 
in tetrahybrid, 94 
in trihybrid 93 
selection, 577 
in fowls, 498 
limitations of, 578, 579 
Phlseum pratense, see Timothy. 
Phylloxera, "benignant" and "malignant" races, 
405 
carysecaulis, 211 
resistance dominant, 408 
factors, 409 
nature of, 404 
vastatrix, 402 
Phylloxeran, grape, 402 

hickory, 211 
Phylogeny and chromosomes, 264 

of the chromosomes, 264 
Physiological, behavior of graft-hybrids, 382 
characters of corn grain, 333 
variations, 16, 18 
Phytomeres, modifications of, 385 

selected, 386 
Pierson, Boston fern, 312 
Pig, see Swine. 
Pigeons, reversion in, 171 

sex-determination in, 545 
sex-linked characters, 209, 539 
Pigmentation, and egg production in fowls, 506 



668 



INDEX 



Pigmentation, extent of, affected by modifying 
factors, 190 

in Drosophila and Leptinotarsa, 261 

modification of, in Coleus, 389 
Pine, Fi hybrids in, 365 
Pineapple, heterosis in, 364 
Piper, alfalfa, 296 

environment factors, 433 

Florida velvet bean, 367 

variety testing, 435 
Pisum sativum, 80 

linkage in, 125 
Plane, London, an Fi hybrid, 365 
Plant breeding, beginning of, 287 
material, 299 
methods, 419 

classified, 291 
organization of, 299 
pioneers in, 288, 437 
progress, 291 
relation of science to, 437 
to genetics, 440 

breeders, organization of, 300, 301 

disease, 400 

groups, research on, 300 

improvement, earliest, 287 

introduction, 299 

populations, composition of, 317 

-to-row method, 337, 433 
Planting, methods of, 420 
Plants, crop, 288, 289, 302 

cross-fertilized, 317 

inheritance of sex. 212 

moncBcious and dioecious, 317 

reproduction in, 317 

self-fertilized, 317 

varieties in, 302 
Plough, crossing-over, temperature effect on, 120, 

121 
Plumcot, 297 
Plums, aphis-resistant, 297 

breeding, 297 

collections, 299 

hybridization, 311 
Pollen, grains of hybrids non-functional, 237 

preservation of, 350 

protection of, 343 

transportation, 300 

viability, 300 
Pollination, artificial, 343 

natural methods of, 317 
Polygamous plants, 318 

Polyphyletic origin of domestic animals, 460 
Pomace fly, see Drosophila ampelophila. 
Pomology, importance of bud selection, 391 
Poplar, Black Italian, an/'i hybrid, 365 
Poppy chimeras, 381 
Population, 37 

containing biotypes, 260 

nature of variations in, 317 
Populations, as affected by crossing, 821 

composition of plant, 317 

creating at Svalof, 352 

normally self-fertilized, 320 



Populus, canadensis-trichocarpa, graft-hybrids, 
378 

Fi hybrids in, 365 

generosa, a vigorous hybrid, 365 
Potato, at Svalof, 293 

bud selection, 394 

certified seed, 397 

clonal selection, 298 

graftage of, 374 

heterosis in, 364 

ideal market tuber, 397 

Magnum Bonum, 290 

mutations in, 369 

running out or degeneration, 395 

sudden degeneration of, 397 

tuber-unit method, 395 
Potency of sex-factors, 217 
Poultry, see Fowls, ducks, etc. 
Powell, apple, 393 
Practical breeding, service of genetics to, 449 

importance of pure varieties, 341 

value of crossing corn, 361 
Predisposition to disease, inheritance of, 523 
Prepotency, fact of, 573 

factor potency interpretatign, 575 

greater in male, 576 

hereditary complex interpretation, 575 

Mendelian interpretation, 574 

recognized, 501 
Presence and absence hypothesis, 153 
Primary non-disjunction, 199 
Primula, hybridization by Dutch, 287 

kewensis, chromosomes, 264 

sinensis rubra, 26, 134 
Primrose, Evening, see CEnothera. 

linkage in Chinese, 125 
Principles of composition of populations, 324 
Pringle, hybridization, 292 

wheat rust, 411 
Pritchard, experimental error, 430 

sugar beet, 340, 369 
Probable error, 45 

experimental determination of, 430 

of a difference, 47 

significance of, 47 
Problems of animal breeding, 447 

of genetics, 4 
Progeny test, 291, 293 
Progress in plant breeding, 291 
Progression method of computing F2 genotypes, 

97 
Progressive mutations in Boston fern, 315 
Promiscuous crossing, results of, 343 
Propagation of mutations, 372 
Protection, of cultures, methods of, 420, 421 

of developing seed, 344 

of pollen in hybridization, 343 

of pollinated flowers, 344 

special devices, 344 
Proteins, specificity of, 266 
Protoplasm, problem of nature of, 268 
Protoplasmic constituents, molecular structure, 

266 
Prunus, research on, 300 



INDEX 



669 



•Przibram, species hybrids, 228 
Psychological variations, 17 
Pucci, Gujarat bull, 531 

zebu cattle, 527 
Puccinia asparagi, 415 
biologic forms, 412 

gluniarum, triticina, and graniinis, 412 
rubigo-vera tritici, 416 
Pumpkin. 317 

Punnett, checkerboard method, 98 
fowl, 209 
linkage in peas, 125 

theory of, 126 
multiple allelomorphs, 160 
presence and absence hypothesis, 153 
rabbit, 225 
Punnett and Bailey, fowls, 500 
Pure-bred livestock, per cent, of, 450 

sires recommended, 510 
Pure line, concept and maize, 293 
method invented, 291 
theory and breeding, 259 
lines, 8, 250 

conditions necessary for, 255 
defined, 255 
discovery of, 250 

effect of selection in, 253, 257. 339 
impossible in allogamous species. 256 
in oats, barley, peas, beans, 340 
in self-fertilized populations, 321, 336 
in wheat, 336 
isolation of, 256 
selection in oats. 371 
Pyrus, communis, sinensis, ovoidea, variolosa, 
betulifolia, 407 
-Cydonia graft-hybrid, 378 



Quantitative inheritance facts of explained, 192, 
194 

factor relations in, 174 

most common type of, 179 

multimodal curves in F2, 179, 181 

must be analyzed, 440 
Quince-pear graft-hybrid, 378 

R 

Rabbits, multiple allelomorphism in, 160, 225 
Race horses, ten greatest, 497 
Rapid growing trees, 365 
Rasmuson, phylloxera-resistance, 408 
Ratio of phenotypes, in dihybrid, 82 

in F2 with w pairs of factors, 98 

in Ft of dihybrid, 85 

in. monohybrid, 71 

in tctrahybrid, 94 

in trihybrid, 93 
Ratios, forces tending to disturb Mendelian. 102 

methods of testing Mendelian, 100 

peculiar Mendelian, 132, 137, 139, 140, 141, 
142 

sex, 539, 542, 546 

standard deviation of Mendelian. 100 



Rats, crossing-over in male. 576 

effects of castration in. 550 

hooded. 186. 187 

linkage in. 125 

sex-ratio in. 542. 547 
Ravaz. phylloxera-resistance. 405 
Reaction system, concept, applied, 575 

Goodspeed and Clausen, 242, 248 
Jesenko, 238 
hypothesis, 242, 248 
systems, in OSnothera hyVjrids, 286 
in species. 242 
Recessive allelomorphs, real factors, 155 
Recombination, 57, 73, 81, 185, 220, 224 

and animal breeding, 515 

and variation, 15, 99, 459 

gametes, 244, 249 

in Nicotiana, 242 

mechanics of, 460 
Recording data, 603 

methods of, 422 

system of, 423 
Red-eye color, in canaries, 209 

in Drosophila, 156, 192 
Redfield, dynamic evolution, 483 
Reduction division, 60, 67, 83, 91 
Reduplication theory of linkage, 126 
Registration requirements for trotting horses, 496 
Regression, 55 

during return selection in hooded rats, 188 

in pure lines, 252 

law of filial, 5 

straight line, 56 
Reichert, specificity of proteins, 266 
Reid, Miss, variegated forms. 382 , 

Reimer, blight, 407, 411 
Reitz, correlation, 458 
Reitz and Smith, effects of selection, 334 

probable error, 47 
Relation, of genetics and plant breeding, 440 

of science to plant breeding, 437 

of yield to feed cost, 446 
Relationship, coefficient of, 600 
Remnant system of maize breeding, 335 
Renner, (Enothera crosses, 247 

CEnothera, seed sterility in, 283 
Reproduction in plants, 317 
Repulsion factor, 126 
Requisites to reliability, 37 

to success in animal breeding, 451 
Research on plant groups, 300 
Resemblances between organisms, 1 
Residual errors, 425 
Reversion, factor explanation of, 170 

in fowls, 171 

in pigeons, 171 
Rhododendron, 438 

hybrid varieties, 311 
Rhubarb, heterosis in, 364 
Rice, 317 

ancient improvement, 287 

imperial Chinese, 287 

little natural crossing, 318 

water requirement. 402 



670 



INDEX 



Riddle, sex control, 545 
Ridgway, color standards, 271 
Riley, telegony, 564 
Rimpau, broad breeding, 335 

hybridization, 294 

Schlanstedt rye, 292 
Ritzman, sheep, 463 
Roberts, turkey wheat, 336 

immediate effects of crossing, 360 
Rogers, grape hybridizer, 290 
Roguing, practical value of, 418 
Role of, factor mutations in evolution, 286 

selection in breeding, 333 
Rommel, telegony, 568, 569, 570 

zebra hybrids, 520, 569 
Root crops, species and varieties, 302 
Rdrig, species hybrids, 228 
Rosa, see Rose. 

RosanofF, hereditary taints, 526 
Rose, creation of varieties, 310 

hybridization by Romans, 287 

My Maryland, 398 

species and varieties, 302, 310 
Roses, bud selection in, 398 
sports in, 398 

hybrid, 311 

Hybrid Bourbons, 311 

Hybrid Chinese, 311 

Hybrid Perpetual, 311 

Hybrid Tea, 311 
Roux, chromatin forms, 115 
Rubber tree, dimorphic branches, 275 
Rubus, species hybrids, 297 
Rudimentary wings in Drosophila, 554 
von Rumker, inbreeding rye, 319 
Running out of varieties, 340 
Rust, biologic forms of, 412 

-resistant asparagus, 415 
wheat, creating, 411 
Rye, 317, 319 

crossed with wheat, 229, 236 

eight chromosomes, 238 

line selection, 293 

remnant system, 335 

reputed self-sterility, 319 



S 



Sageret, species hybrids, 230 
Salmon, Erysiphe, 413 
Salsify, first hybridized, 438 
Salt bushes, Australian, 401 

grass, 401 
Sambucus glauca, 17 

nigra variegata, 381 
Sanborn, increased yield from hybrids, 354 
Saturation, 571 

Saunders, A. P., Marquis wheat, 294 
Saunders, Miss, stocks, inheritance in, 166, 167 
Saunders, Wm., apples, 297 

cold resistant fruits, 408 
Scardafella inca, 24, 25 
Science and plant breeding, 437 
Scilla, 25 



Scion and stock, reciprocal effects, 382, 383 

Scrophularia, 27 

Scrub cows, records of, 455 

Search for mutations, 370, 464 

Secondary, non-disjunction, 200 

sexual characters, 214, 548 
Sectorial chimera, 375, 376, 380 
Sedum spectabile, 21 
Seed, corn, centralized production, 361 
hybrid, 361 

impurity of commercial, 428 

introduction, 299 

selection, value of, 335 
Seedage, methods of, 419 
Segregation, 57, 64, 67, 81, 94, 108, 188, 453, 515 

discovered by Naudin, 439 

evidence of, in quantitative inheritance, 181 

in species hybrids, 220, 224 

mechanism, 61, 274 

of size factors, 184 

somatic, a misleading term, 275 
vs. vegetative mutation, 272, 274 
Selby and Houser, tobacco, 363 
Selection, 11, 250, 325 

and breeding methods, 500 

and correlation, 49, 506 

and variation, 325 

as a cause of variation, 454 

bud, 298, 385, 386, 391 

changes position of mean, 188 

clonal, 291, 298 

ear-to-row, 327, 334 

effect of, in pure lines, 254, 257, 339 

for disease resistance, 416 

genotypic, 262, 425, 496, 584 

guided by progeny tests, 498 
by registration rules, 496 

in creating varieties of plants, 302 

in maize breeding, 325, 333, 334 

in sweet peas, 309 

in wheat, 336 

indices, 334, 502 

individual plant, 340 

isolation view of, 502 

line, 291 

mass, 260, 291, 498 

natural and evolution, 5, 276, 286 

not creative, 502 

of disease-resistant strains, 417 

of phenotypes may not isolate genotypes, 261 

of profitable strains, 432 

phenotypic, 578 

plant-to-row method, 337 

plus and minus in hooded rats, 191 

problem in animal breeding, 495 

remnant system, 335 

results of, in hooded rats explained, 191, 192 

two views regarding, 495 

value of seed, 335 
Self-fertilized plants, selection in, 336 

populations of plants, 320 
Self, colors, mutations, 272, 273 

-fertilization, effects of, 256, 320 
Semen, "Mnemetheorie," 483 



i 



INDEX 



671 



Separate culture, 293 

Serum albumin, stereoisonieric foriiis, 266 

Service, of genetics, 448, 450, 609 

time of and sex-ratio, 543 
Sesqui-hybrid, 73 

combinations, 235. 237, 244, 248 
Sex-chromosomes, 58 66, 116, 211, 546 

-factors, 215 

-hormones. 545, 550 

in animals, 536 

-inheritance of, 65, 196, 204, 205, 211 

potency of factors, 217 
Sex-determination, and genotypic constitution, 
218 
in animals, 536 
in honey bee, 211 
in hickory phylloxeran, 211 
in insects, 210 
in man, 538 
in mullein thrips, 211 

metabolic theories of, 544 
Sexes, numerical equality of, 67 
Sex-inheritance, 65 

in plants, 212 

non-disjunction and, 204 

WZ type of, 205 

Xy type of, 196 
Sex-linked characters, 197, 208, 209, 538 
in a plant, 214 

factors, 117 

in X-chromosome, 196, 546 

inheritance, 74 

lethal factors in Drosophila, 132 
Sex-ratios, 196, 539 

causes of unusual, 542 

inheritance of, 546 

in man, 540 • 
Sexual characters, secondary, 214, 548 
Sexuality in plants established, 437 
Shamel, bud selection, 391 

maize, 292 
Sharp, apple breeding, 295 
Shaw and Heller, horns in sheep, 549 
Sheep, Ancon, 462 

Corriedale, 584 

crossbreeding, 515 

diversity of ancestry, 460 

Dorsett, 515, 549 

effect of castration, 549 

horns in, 214, 549 

marking, 602 

Mendelism in, 475 

Merino, 515 

mutations in, 462, 463 

Rambouillet, 463 

sex-determination in, 536 

Shropshire, 463 

Southdown, 463 

sterility in, 552 

Suffolk, 549 

telegony, 564 
Shirreff, cereals, 289 

plant breeding, 437 

selection, 366 



Shull, A. F., mullein thrips, 211 
Shull, G. H., acquired characters, 480 

Bursa, 421 

corn breeding, 356 

factors multiple, 139 

genotypes in maize, 326, 329 

heterosis, 231, 353 

inbreeding in maize, 325 

increased yield from hybrids, 353 

Lychnis dioica, 213, 214, 215 

maize, 293, 295 

multiple allelomorphs in Lychnis, 150 

records, 422 

regression, 56 

variation, 369 
Silkworm, larval pattern, 210 

linkage in, 125, 209 

multiple allelomorphs in, 158, 210 
Size, a complex character, 194 

inheritance of, 174, 188 
Skew polygons, 35 

Small grains, composite crossing of, 296 
Smith, L. H., corn breeding experiments, 327, 329 

maize, 292 
Smith and Thomas, pheasants, 558 
Smut fungus, immunity of milo, 416 
Snapdragon, 440 

common, 90, 133 

dimorphic flowers, 275 

variegated, 381 
Soil heterogeneity, correcting for, 431 

criterion of, 430 
Solanum chimeras, 374, 375 

Darwinianum, 377 

dulcamara, 378 

Gartnerianum, 375, 377, 382 

Kolreuterianum, 375, 377, 382 

lycopersicum, 375, 378 

melongena, 378 

nigrum, 375, 378 

proteus, 375, 377, 382 

tubigense, 375, 376, 382 
Soma and germ-plasm, 2, 485, 490 
Somatic cell division, 59 

modifications, 453 

mutations, 386 

segregation a misleading term, 275 
vs. vegetative mutation, 272, 274 

variations, 272 
Somatogenesis, 2, 20 
Somatogenic characters, 480 
Sorghum, collecting stock seed, 341 

origin of varieties, 370 
Southworth, alfalfa, 297 
Species, disease resistance in, 401 

factors in, 219 

homologous factors, 224 

hybridization, 219, 310, 312, 342 
in domestic animals, 515 

hybrids, forms of, 227 

peculiar phenomena in, 249, 542 
steriUty in, 234 
vigor of, 230 

in (Enothera, 244, 280 



672 



INDEX 



Species, peculiarities in hybridization, 342 

universal indicator, 234 
Specificity of proteins and carbohydrates, 266 
Spencer, acquired characters, 6, 480 
germinal theory, 489 
on Darwin, 5 
Sperm, male and female producing, 196, 540 
Spermatozoa, dimorphism in, 540 

in Equus hybrids, 520 
Spillman, polled cattle, 463 

swine, 476 
Spindle, 60, 61, 64 
Spindle Tree, chimeras in, 381 
Spireme, 59 
Sports, bud, 270, 298 
in sweet peas, 304 
utilization of, 302 
Spragg, alfalfa, 417 

barleys, 340 
Sprengel, hybrids, 438] 
Squash, 317 

Standard deviation, calculation and significance, 
41 
diminished during selection, 188 
of a Mendelian ratio, 100 
of an Fi population, 184 
Standardization of varieties, 432 
Stapels-Brown, reversion in pigeons, 172 
Starch grains, in round and wrinkled peas, 145 

stereoisomeric forms of, 266 
Starkman and Piemeisel, wheat rust, 412 
State associations of breeders, 301 
Statistical method, employment and value of, 56 
Steinach, ovarian transplantation, 550 
Stereoisomeric forms of organic substances, 266 
Sterility of hybrids, 556 
in animals, 555 

in species hybrids, 227, 228, 234, 236, 238 
Stewart, degeneration in potatoes, 397 
St. Hilaire, fertility, 438 

Stockard, effect of alcohol on guinea-pigs, 30, 493 
Stock and scion, reciprocal effects, 383 
Stockberger, experimental error, 430 
Stocks, see Matthiola. 
von der Stok, intergeneric hybrids, 296 
Stout, Coleus, 386, 387, 388, 389 
Strain tests at Svalof, 425 
test of tomatoes, 433 
tests, purposes, 432 
Strains, disease resistant, 417 
Strawberry, breeding in America, 289, 294 
bud selection in, 398 
heterosis in, 364 
hybridization, 311, 398 
Strawberry Bush, see Spindle Tree. 
Stuart, bud selection, 394, 395 
potato, 428 
tuber-unit, 396, 397 
Sturtevant, crossing-over, factors that lower, 120, 

210 
Sturtevant and Bridges, crossing-over, 113 

no crossing-over in the male, 114 
Substitutional changes in factors, 267 
Sugar beet, 317 



Sugar beat, improvement begun, 291 
line selection, 340 
mass selection, 292, 340 
selection of mutations, 340, 370 
cane, heterosis in, 364 
Sumner, acquired characters, 491 
Sunflower, 317, 318 
chimeras, 381 

isolation of pure strains, 257 
red, 270 
self-sterile, 257 
Supernumerary legs in Drosophila, 136 
Surface, corn selection, 329, 330, 333 

linkage in oats, 125 
Surface and Barber, experimental error, 430 

oats, 429 
Surface and Pearl, soil heterogeneity, 431 
Surface and Zinn, oats, 340, 371 
planting board, 339 
record blanks, 435 
variations, 369 
Survival of the fittest, 5 
Sutton, plant breeding, 437 

plant improvements, 290 
Svalof, method, 352 

plant breeding station, 258, 293, 340 
system of breeding, 425 
Swedish, plant breeding station, 258 

Seed Association, 293 
Sweet pea, Blanche Ferry group, 309 
bush, 175, 308 
bush-Cupid, 176 
contabescence, 234 
collections, 299 
complementary factors, 140 
Cupid, 175, 177, 308 
diversity of variety, 303 
double, 307 
early flowering, 309 
first recorded hybrid, 309 
flower color in, 394 

form and size, 304 
habit in, 308 
history, 303, 304, 305 
hybridization and selection, 309 
linkage in, 125 
mutations in, 304 
novelty forms, 307 
origin of varieties, 303, 305 
snapdragon, 307 
species and varieties, 302 
tall and dwarf, 174, 175 
waved or Spencer, 306 
Swine, chromosome number, 537 
Hampshire, 476 
Mendelism in, 476 
mule footed, 476 
sex-determination in, 536 
spermatozoa in, 540 
white dominant in, 579 
Swingle, citrus, 297 
Synapsis, 61, 67, 108, 115, 121, 123, 199, 201, 204, 

559 
System, of aleurone color factors in maize, 163 



4 



I 



1 



INDEX 



673 



System, of labelling and recording data, 423 

of pedigree, 293 

of reaction, in species, 242, 286 

Svalof , 425 
Systems of breeding, 580 



Tanaka, linkage relations, 209, 210 

multiple allelomorphs in silkworms, 158 
Telegony, 564, 570 
Teosinte, crossed with maize, 296 
Terminology, Mendelian, 71 
Tetrahybrid ratio, 94 
Tetraploidy, 263 

causing gigantism, 264 
Thaer, plant breeding, 437 

selection, 288 
Theory of error, 44 

of evolution, de Vries', 282 

of multiple factors in size inheritance, 185 

of reduplication in linkage, 126 
Thomson, acquired characters, 481, 491, 494 

heredity concept, 3 
Tick, Texas fever, 528 
Tilia vulgaris, 365 

Timber trees, rapid growing hybrid, 365 
Timothy, breeding, 294 

remnant system, 335 
Tinged eye color in Drosophila, 156 
Tissues, in graft-hybrids, 377, 380 
Tobacco, 317 

corolla length in, 181 

crossed by humming birds, 319 

decreased vigor in some species hybrids, 230 

disease resistant strains, 418 

fasciated, 130 

graftage of, 374 

increased yield in Fi, 363 

mutation affecting stature, 177 

mutations in, 367 

Stewart Cuban, 368 
Tomato, 317 

chimeras, 381 

color chimera, 272 

higher yield in Fi, 362 

mutations in, 369 

nightshade graft-hybrids, 374 

practicability of using Fi seed, 363 

red cherry, 28, 29 

strain tests, 432 

tall and dwarf, 174 

wilt resistant, 415 
Tower, beetles, 261, 259, 370 

germinal variations, 30, 31 

Leptinotarsa, 248, 259 

pure strain, 259 

selection effect, 258, 260 
Tragopogon pratensis X T. porrifolius, 438 
Training, question of inheritance of, 480, 483 
Trapnesting hens, 592, 593 
Trees, "pedigreed" vs. recorded, 394 

rapid growing hybrid, 365 

top working poor, 391 
43 



Trial gardens, 299 
Trihybrid. 90 

ratio, typical, 93 
Triploidy, 263 
Triticum, vulgare var. fuliguosum, 412 

vulgare, sub-species, 339 
Tropteolum majus X T. minus, 230 
Trotting, horses, registration of, 496 

record, reduction of, 484, 495 
Trow, linkage theory, 126 
Truncate-winged Drosophila, Altenburg and 

Muller's, 168 
von Tschermak, rediscovery of Mendel's prin- 
ciples, 68 
Tuber-unit method in potato, 395 
Tufts, somatic segregation, 274 
Tulip, hybridization by Dutch, 287 

species and varieties, 302 
Type, building the ideal, 611 

maintaining by vegetative propagation, 386 
Types, determining ideal, 612 

difficulties in establishing, 428 

of factor interactions, 163 

originate by mutation, 386 
Tyson, York Imperial apples, 394 
Tyzzer, mice, 529 

tumors, 527 



U 



Uniformity in Fi of species hybrids, 227 

Unit characters, 71, 72 

Upton, horses, 444 

Urban, alfalfa, 296 

Use and disuse, inheritance of effects of, 492 

Utilization of clonal diversity, 302 
of hybrids in breeding, 353 
of locally adapted plants, 441 
of mutations, 302, 464 



Valk, grape hybridizer, 290 
Vanessaio, 23 
Van Fleet, chestnuts, 410 

Variability, increased on crossing selected races, 
188 

in Fa in quantitative inheritance, 186 ] 

Variable character expression in hybrids, 149 
Variation, 1, 2, 439 

and development, 20 

and environment, 21 

bud, 385 

by recombination, 459 

caused by modifiability, 454 
by selection, 454 

complexity of, 428 

concept, 15 

defined, 3 

fluctuating, 16, 18, 32 

importance of understanding, 325 

in domestic animals, 453 

universality of, 14 



674 



INDEX 



Variations, classification of, 15 
autogenetic, 20 
combinations, 15 
continuous, 18 
differences between them, 18 
direction, 20 
discontinuous, 18, 274 
ecological, 18 
ectogenetic, 20 
fluctuating, 18, 32 
fortuitous, 20 
heritability, 15 
homeotic, 16 
meristic, 16 

minus and plus in hooded rats, 188 
modifications, 15 
morphological, 16, 17 
mutations, 15 
nature, 16, 317 
orthogenetic, 20 
physiological, 16, 18 
psychological, 17 
Variegated, Abutilons, 382 
geranium, 381 
maize, 272, 381 
MirabUis, 273, 381 
plants not mosaics, 273 
sporting of, 272, 381 
snapdragon, 381 
Variegation, association with mutation, 272 
factor mutations, 381 
in Malvacese, 381 
pathological, 381 
Varieties, disease resistance in, 416 
factors in, 219] 

importance of keeping pure, 341 
in plants, 302 
methods of creating, 302 
origin of, in Boston fern, 312 

of sweet pea, 303 
running our, 340, 395 
standardization of, 432 
value of proved sorts, 341 
Variety testing, experimental errors, 429 
standards for, 435 
stations, 416 
tests, 427 j 
Vavilov, rust resistant wheat, 412 
Vegetative, in Boston fern, 312, 316 

mutation vs. somatic segregation, 272 
mutations, 270 
Verbascum, 438 

blattaria X V. lychnitis, 230 
lychnitis X V. thapsus, 230 
Vestigial wings, inheritance of, in Drosophila, 87 
Viala and Ravaz, grape phylloxera, 403 
Vigor in Fi hybrids, 230, 326 
de Vilmorin, Andr6, carrots, 290 

mass selection, 292 
de Vilmorin, Louis, pedigree culture, 419 
linkage in peas, 125 
plant breeding, 299, 437 
progeny test, 293 
seed establishment, 290 



de Vilmorin, selection, 366 

sugar beet, 291 

wheat, 257, 258, 291, 419 
Vilmorin method of plant breeding, the, 291, 293 
Vine, see Grape. 
Violet, bud selection, 391 

bud sports in, 398 

dimorphic flowers, 275 

species and varieties, 302 
Vitis, cinerea, aestivalis, linsecomii, candicans, 
rotundifolia, californica, rupestris, 
riparia, cordifolia, and verlanderi, 404 

vinifera, 403 
de Vries, Alhambra plum, 298 

"Die Mutationstheorie," 282 

discovery, 439 

evolution theory, 276 

mutants, 279 
classified, 282 

mutation theory, 3, 7, 371 

mutations, 283, 286 
discovery of, 276 

CEnothera crosses, 229, 244 

CEnothera lamarckiana, 277 

(E. laivifolia, 278 

on Le Couteur, 289 

on Shirreff, 289 

pangens, 246, 247 

progressive species, 281 

rediscovery of Mendel's principles, 68 

somatic variations, 272 

species hybridization, 244 

W 

Wagner, immunity to rust, 413 
vonWahl, mare mule and foal, 518 
Walnut, heterosis in, 364 

oak-like, 265 

Paradox, 364 

Royal, 297, 364 
Warner, egg production, 507 
Wasps destroy paper bags, 344 
Watermelon, 317 

wilt resistant hybrid, 414 
Waugh, on grape hybridizers, 290 
Webber, citrus, 297 

clonal or bud selection, 391 

corn, 293 

cotton, 292 

tuber-unit, 395 
Weismann, acquired characters, 480, 484 

amphimixis, 3, 484 

body and strip, 489 
Wellington, tomato, 362, 363 
Wentworth, color in horses, 466, 467, 468 
Westgate, alfalfa, 296 
Wheat, 317 

autogomaous, 257 

breeding begun, 289 

centgener method, 293 

crossed with rye, 229, 236 

Defiance, 336 

duplicate factors discovered in, 137 



INDEX 



675 



wheat, eight chromosomes, 238 

Ghirka, 372 

Gladden, 339 

Glorup, 257 

Gypsy, 337, 338 

hybridization of, 294, 345 

inflorescence, 345, 346 

maintenance of type, 257 

mutations in, 369 

natural hybrids in, 348 

pure lines, 293, 336, 372 

rust-resistant, creating, 411 
varieties, 416 

sometimes cross-fertilized, 318 

Turkey, 336 

"Vilmorin Method," 291 

Yellow-berry, 418 
Whedale, Miss, Antirrhinum, inheritance in, 147 
White, O. E., chimeras, 376 

fasciated plants, 130 

linkage in Pisum, 125 
White, T. H., germinal variations, 28, 370 
White eye color, inheritance of, in Drosophila, 74, 
87, 197, 208 

factor decreases vigor, 233 
mutation, 267, 269 
White eyed mutants, discovery, 74, 156 
Whitten, Ben Davis apple, 394 

bud selection, 398 
Whitethorn-medlar chimera, 378, 379 
Whymper, cretins, 526 
Wickson, on Burbank, 298 
Wieman, spermatogenesis in man, 538 
Williams, Gladden wheat, 339 

Gypsy-wheat, 337, 338 

remnant system, 334 
Willow, Cricket-bat, an Fi hybrid, 365 
Wilson, E. B., heredity defined, 57 

problems, of inheritance, 4 
Wilson, E. H., rose varieties, 310 

seed and plant introduction, 299 
Wilson, J., color in horses, 467, 469, 470, 472, 473 

Dexter-Kerry cattle, 627 

Morton's quagga, 566 

multiple allelomorphs, 474 

sheep, 476 
Wilt diseases, 413 

-resistant hybrids, 414, 415, 416, 417 
von Winiwarter, chromosomes in man, 538 
Winkler, chimeras, 375, 376, 377, 378 

graft-hybrids, 374 



Wodsedalek, chromosomes, horse, 519, 537 

determination of sex, 536 

sperm of mule, 558 
of pig, 540, 541 
Woltereck, effect of food supply, 23 

Hyalodaphnia, 23 
Wood, sheep, 549 

Wood and Stratton, experimental error, 430 
Work, varieties, 432 
TT-chromosome, 205, 209 
Wriedt, Norway sheep, 463 
WZ type of sex-inheritance, 205 

animals showing, 209 
in birds, 539 



X 



X-chromosome, 58, 74, 88, 110, 112, 114, 117, 119, 

121, 155, 196, 214, 267 
XY type of sex-inheritance, 196 

animals showing, 196, 214 

ii. domestic animals, 536 



Yak crossed with domestic cow, 520 
y-chromosome, 58, 74, 88, 112, 114, 196, 214, 234 
Yields, method of comparing, 356 



Z-chromosome, 205, 209 
Zea, hirta, 358 
hybrids, 359 
mays, 358 

amylacea, 358 
everta, 358 
indentata, 358 
indurata, 358 
Zebra-ass hybrid, 557 
Grevy, 568 

-horse crosses, 468, 520, 557, 567, 570 
quagga-horse hybrids, 565 
Zebu cattle, 460, 461 

disease immune, 628 
for grading, 529 
Gujarat, 530 

X domestic cow, 520, 531 
Zenia, advantage of, in experimentation, 360 
cause of, 150 



/ 



u ■/, 



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